diff --git a/composer.json b/composer.json
index ed4f5e8f..7ffa2737 100644
--- a/composer.json
+++ b/composer.json
@@ -13,12 +13,16 @@
         }
     ],
     "require": {
-        "php": "^7.0"
+        "php": "^8.2",
+        "vlucas/phpdotenv": "^5.6",
+        "phpoption/phpoption": "^1.9",
+        "ext-mbstring": "*"
     },
     "require-dev": {
         "php-parallel-lint/php-parallel-lint": "^1.2.0",
         "phpcompatibility/php-compatibility": "^9.3.5",
-        "squizlabs/php_codesniffer": "^3.5.5"
+        "squizlabs/php_codesniffer": "^3.9.0",
+        "ext-simplexml": "*"
     },
     "scripts": {
         "lint": "parallel-lint . --exclude vendor",
diff --git a/src/Dotenv/Dotenv.php b/src/Dotenv/Dotenv.php
new file mode 100644
index 00000000..0460ced2
--- /dev/null
+++ b/src/Dotenv/Dotenv.php
@@ -0,0 +1,267 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv;
+
+use Dotenv\Exception\InvalidPathException;
+use Dotenv\Loader\Loader;
+use Dotenv\Loader\LoaderInterface;
+use Dotenv\Parser\Parser;
+use Dotenv\Parser\ParserInterface;
+use Dotenv\Repository\Adapter\ArrayAdapter;
+use Dotenv\Repository\Adapter\PutenvAdapter;
+use Dotenv\Repository\RepositoryBuilder;
+use Dotenv\Repository\RepositoryInterface;
+use Dotenv\Store\StoreBuilder;
+use Dotenv\Store\StoreInterface;
+use Dotenv\Store\StringStore;
+
+class Dotenv
+{
+    /**
+     * The store instance.
+     *
+     * @var \Dotenv\Store\StoreInterface
+     */
+    private $store;
+
+    /**
+     * The parser instance.
+     *
+     * @var \Dotenv\Parser\ParserInterface
+     */
+    private $parser;
+
+    /**
+     * The loader instance.
+     *
+     * @var \Dotenv\Loader\LoaderInterface
+     */
+    private $loader;
+
+    /**
+     * The repository instance.
+     *
+     * @var \Dotenv\Repository\RepositoryInterface
+     */
+    private $repository;
+
+    /**
+     * Create a new dotenv instance.
+     *
+     * @param \Dotenv\Store\StoreInterface           $store
+     * @param \Dotenv\Parser\ParserInterface         $parser
+     * @param \Dotenv\Loader\LoaderInterface         $loader
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     *
+     * @return void
+     */
+    public function __construct(
+        StoreInterface $store,
+        ParserInterface $parser,
+        LoaderInterface $loader,
+        RepositoryInterface $repository
+    ) {
+        $this->store = $store;
+        $this->parser = $parser;
+        $this->loader = $loader;
+        $this->repository = $repository;
+    }
+
+    /**
+     * Create a new dotenv instance.
+     *
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     * @param string|string[]                        $paths
+     * @param string|string[]|null                   $names
+     * @param bool                                   $shortCircuit
+     * @param string|null                            $fileEncoding
+     *
+     * @return \Dotenv\Dotenv
+     */
+    public static function create(RepositoryInterface $repository, $paths, $names = null, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $builder = $names === null ? StoreBuilder::createWithDefaultName() : StoreBuilder::createWithNoNames();
+
+        foreach ((array) $paths as $path) {
+            $builder = $builder->addPath($path);
+        }
+
+        foreach ((array) $names as $name) {
+            $builder = $builder->addName($name);
+        }
+
+        if ($shortCircuit) {
+            $builder = $builder->shortCircuit();
+        }
+
+        return new self($builder->fileEncoding($fileEncoding)->make(), new Parser(), new Loader(), $repository);
+    }
+
+    /**
+     * Create a new mutable dotenv instance with default repository.
+     *
+     * @param string|string[]      $paths
+     * @param string|string[]|null $names
+     * @param bool                 $shortCircuit
+     * @param string|null          $fileEncoding
+     *
+     * @return \Dotenv\Dotenv
+     */
+    public static function createMutable($paths, $names = null, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $repository = RepositoryBuilder::createWithDefaultAdapters()->make();
+
+        return self::create($repository, $paths, $names, $shortCircuit, $fileEncoding);
+    }
+
+    /**
+     * Create a new mutable dotenv instance with default repository with the putenv adapter.
+     *
+     * @param string|string[]      $paths
+     * @param string|string[]|null $names
+     * @param bool                 $shortCircuit
+     * @param string|null          $fileEncoding
+     *
+     * @return \Dotenv\Dotenv
+     */
+    public static function createUnsafeMutable($paths, $names = null, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $repository = RepositoryBuilder::createWithDefaultAdapters()
+            ->addAdapter(PutenvAdapter::class)
+            ->make();
+
+        return self::create($repository, $paths, $names, $shortCircuit, $fileEncoding);
+    }
+
+    /**
+     * Create a new immutable dotenv instance with default repository.
+     *
+     * @param string|string[]      $paths
+     * @param string|string[]|null $names
+     * @param bool                 $shortCircuit
+     * @param string|null          $fileEncoding
+     *
+     * @return \Dotenv\Dotenv
+     */
+    public static function createImmutable($paths, $names = null, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $repository = RepositoryBuilder::createWithDefaultAdapters()->immutable()->make();
+
+        return self::create($repository, $paths, $names, $shortCircuit, $fileEncoding);
+    }
+
+    /**
+     * Create a new immutable dotenv instance with default repository with the putenv adapter.
+     *
+     * @param string|string[]      $paths
+     * @param string|string[]|null $names
+     * @param bool                 $shortCircuit
+     * @param string|null          $fileEncoding
+     *
+     * @return \Dotenv\Dotenv
+     */
+    public static function createUnsafeImmutable($paths, $names = null, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $repository = RepositoryBuilder::createWithDefaultAdapters()
+            ->addAdapter(PutenvAdapter::class)
+            ->immutable()
+            ->make();
+
+        return self::create($repository, $paths, $names, $shortCircuit, $fileEncoding);
+    }
+
+    /**
+     * Create a new dotenv instance with an array backed repository.
+     *
+     * @param string|string[]      $paths
+     * @param string|string[]|null $names
+     * @param bool                 $shortCircuit
+     * @param string|null          $fileEncoding
+     *
+     * @return \Dotenv\Dotenv
+     */
+    public static function createArrayBacked($paths, $names = null, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $repository = RepositoryBuilder::createWithNoAdapters()->addAdapter(ArrayAdapter::class)->make();
+
+        return self::create($repository, $paths, $names, $shortCircuit, $fileEncoding);
+    }
+
+    /**
+     * Parse the given content and resolve nested variables.
+     *
+     * This method behaves just like load(), only without mutating your actual
+     * environment. We do this by using an array backed repository.
+     *
+     * @param string $content
+     *
+     * @throws \Dotenv\Exception\InvalidFileException
+     *
+     * @return array<string,string|null>
+     */
+    public static function parse(string $content)
+    {
+        $repository = RepositoryBuilder::createWithNoAdapters()->addAdapter(ArrayAdapter::class)->make();
+
+        $phpdotenv = new self(new StringStore($content), new Parser(), new Loader(), $repository);
+
+        return $phpdotenv->load();
+    }
+
+    /**
+     * Read and load environment file(s).
+     *
+     * @throws \Dotenv\Exception\InvalidPathException|\Dotenv\Exception\InvalidEncodingException|\Dotenv\Exception\InvalidFileException
+     *
+     * @return array<string,string|null>
+     */
+    public function load()
+    {
+        $entries = $this->parser->parse($this->store->read());
+
+        return $this->loader->load($this->repository, $entries);
+    }
+
+    /**
+     * Read and load environment file(s), silently failing if no files can be read.
+     *
+     * @throws \Dotenv\Exception\InvalidEncodingException|\Dotenv\Exception\InvalidFileException
+     *
+     * @return array<string,string|null>
+     */
+    public function safeLoad()
+    {
+        try {
+            return $this->load();
+        } catch (InvalidPathException $e) {
+            // suppressing exception
+            return [];
+        }
+    }
+
+    /**
+     * Required ensures that the specified variables exist, and returns a new validator object.
+     *
+     * @param string|string[] $variables
+     *
+     * @return \Dotenv\Validator
+     */
+    public function required($variables)
+    {
+        return (new Validator($this->repository, (array) $variables))->required();
+    }
+
+    /**
+     * Returns a new validator object that won't check if the specified variables exist.
+     *
+     * @param string|string[] $variables
+     *
+     * @return \Dotenv\Validator
+     */
+    public function ifPresent($variables)
+    {
+        return new Validator($this->repository, (array) $variables);
+    }
+}
diff --git a/src/Dotenv/Exception/ExceptionInterface.php b/src/Dotenv/Exception/ExceptionInterface.php
new file mode 100644
index 00000000..1e80f531
--- /dev/null
+++ b/src/Dotenv/Exception/ExceptionInterface.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Exception;
+
+use Throwable;
+
+interface ExceptionInterface extends Throwable
+{
+    //
+}
diff --git a/src/Dotenv/Exception/InvalidEncodingException.php b/src/Dotenv/Exception/InvalidEncodingException.php
new file mode 100644
index 00000000..f02f8e45
--- /dev/null
+++ b/src/Dotenv/Exception/InvalidEncodingException.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Exception;
+
+use InvalidArgumentException;
+
+final class InvalidEncodingException extends InvalidArgumentException implements ExceptionInterface
+{
+    //
+}
diff --git a/src/Dotenv/Exception/InvalidFileException.php b/src/Dotenv/Exception/InvalidFileException.php
new file mode 100644
index 00000000..6e878a58
--- /dev/null
+++ b/src/Dotenv/Exception/InvalidFileException.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Exception;
+
+use InvalidArgumentException;
+
+final class InvalidFileException extends InvalidArgumentException implements ExceptionInterface
+{
+    //
+}
diff --git a/src/Dotenv/Exception/InvalidPathException.php b/src/Dotenv/Exception/InvalidPathException.php
new file mode 100644
index 00000000..ee836a73
--- /dev/null
+++ b/src/Dotenv/Exception/InvalidPathException.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Exception;
+
+use InvalidArgumentException;
+
+final class InvalidPathException extends InvalidArgumentException implements ExceptionInterface
+{
+    //
+}
diff --git a/src/Dotenv/Exception/ValidationException.php b/src/Dotenv/Exception/ValidationException.php
new file mode 100644
index 00000000..3d6773af
--- /dev/null
+++ b/src/Dotenv/Exception/ValidationException.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Exception;
+
+use RuntimeException;
+
+final class ValidationException extends RuntimeException implements ExceptionInterface
+{
+    //
+}
diff --git a/src/Dotenv/Loader/Loader.php b/src/Dotenv/Loader/Loader.php
new file mode 100644
index 00000000..d8e194cc
--- /dev/null
+++ b/src/Dotenv/Loader/Loader.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Loader;
+
+use Dotenv\Parser\Entry;
+use Dotenv\Parser\Value;
+use Dotenv\Repository\RepositoryInterface;
+
+final class Loader implements LoaderInterface
+{
+    /**
+     * Load the given entries into the repository.
+     *
+     * We'll substitute any nested variables, and send each variable to the
+     * repository, with the effect of actually mutating the environment.
+     *
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     * @param \Dotenv\Parser\Entry[]                 $entries
+     *
+     * @return array<string,string|null>
+     */
+    public function load(RepositoryInterface $repository, array $entries)
+    {
+        return \array_reduce($entries, static function (array $vars, Entry $entry) use ($repository) {
+            $name = $entry->getName();
+
+            $value = $entry->getValue()->map(static function (Value $value) use ($repository) {
+                return Resolver::resolve($repository, $value);
+            });
+
+            if ($value->isDefined()) {
+                $inner = $value->get();
+                if ($repository->set($name, $inner)) {
+                    return \array_merge($vars, [$name => $inner]);
+                }
+            } else {
+                if ($repository->clear($name)) {
+                    return \array_merge($vars, [$name => null]);
+                }
+            }
+
+            return $vars;
+        }, []);
+    }
+}
diff --git a/src/Dotenv/Loader/LoaderInterface.php b/src/Dotenv/Loader/LoaderInterface.php
new file mode 100644
index 00000000..275d98e8
--- /dev/null
+++ b/src/Dotenv/Loader/LoaderInterface.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Loader;
+
+use Dotenv\Repository\RepositoryInterface;
+
+interface LoaderInterface
+{
+    /**
+     * Load the given entries into the repository.
+     *
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     * @param \Dotenv\Parser\Entry[]                 $entries
+     *
+     * @return array<string,string|null>
+     */
+    public function load(RepositoryInterface $repository, array $entries);
+}
diff --git a/src/Dotenv/Loader/Resolver.php b/src/Dotenv/Loader/Resolver.php
new file mode 100644
index 00000000..36d7a4b9
--- /dev/null
+++ b/src/Dotenv/Loader/Resolver.php
@@ -0,0 +1,65 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Loader;
+
+use Dotenv\Parser\Value;
+use Dotenv\Repository\RepositoryInterface;
+use Dotenv\Util\Regex;
+use Dotenv\Util\Str;
+use PhpOption\Option;
+
+final class Resolver
+{
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Resolve the nested variables in the given value.
+     *
+     * Replaces ${varname} patterns in the allowed positions in the variable
+     * value by an existing environment variable.
+     *
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     * @param \Dotenv\Parser\Value                   $value
+     *
+     * @return string
+     */
+    public static function resolve(RepositoryInterface $repository, Value $value)
+    {
+        return \array_reduce($value->getVars(), static function (string $s, int $i) use ($repository) {
+            return Str::substr($s, 0, $i).self::resolveVariable($repository, Str::substr($s, $i));
+        }, $value->getChars());
+    }
+
+    /**
+     * Resolve a single nested variable.
+     *
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     * @param string                                 $str
+     *
+     * @return string
+     */
+    private static function resolveVariable(RepositoryInterface $repository, string $str)
+    {
+        return Regex::replaceCallback(
+            '/\A\${([a-zA-Z0-9_.]+)}/',
+            static function (array $matches) use ($repository) {
+                return Option::fromValue($repository->get($matches[1]))
+                    ->getOrElse($matches[0]);
+            },
+            $str,
+            1
+        )->success()->getOrElse($str);
+    }
+}
diff --git a/src/Dotenv/Parser/Entry.php b/src/Dotenv/Parser/Entry.php
new file mode 100644
index 00000000..7570f587
--- /dev/null
+++ b/src/Dotenv/Parser/Entry.php
@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+use PhpOption\Option;
+
+final class Entry
+{
+    /**
+     * The entry name.
+     *
+     * @var string
+     */
+    private $name;
+
+    /**
+     * The entry value.
+     *
+     * @var \Dotenv\Parser\Value|null
+     */
+    private $value;
+
+    /**
+     * Create a new entry instance.
+     *
+     * @param string                    $name
+     * @param \Dotenv\Parser\Value|null $value
+     *
+     * @return void
+     */
+    public function __construct(string $name, Value $value = null)
+    {
+        $this->name = $name;
+        $this->value = $value;
+    }
+
+    /**
+     * Get the entry name.
+     *
+     * @return string
+     */
+    public function getName()
+    {
+        return $this->name;
+    }
+
+    /**
+     * Get the entry value.
+     *
+     * @return \PhpOption\Option<\Dotenv\Parser\Value>
+     */
+    public function getValue()
+    {
+        /** @var \PhpOption\Option<\Dotenv\Parser\Value> */
+        return Option::fromValue($this->value);
+    }
+}
diff --git a/src/Dotenv/Parser/EntryParser.php b/src/Dotenv/Parser/EntryParser.php
new file mode 100644
index 00000000..e286840a
--- /dev/null
+++ b/src/Dotenv/Parser/EntryParser.php
@@ -0,0 +1,300 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+use Dotenv\Util\Regex;
+use Dotenv\Util\Str;
+use GrahamCampbell\ResultType\Error;
+use GrahamCampbell\ResultType\Result;
+use GrahamCampbell\ResultType\Success;
+
+final class EntryParser
+{
+    private const INITIAL_STATE = 0;
+    private const UNQUOTED_STATE = 1;
+    private const SINGLE_QUOTED_STATE = 2;
+    private const DOUBLE_QUOTED_STATE = 3;
+    private const ESCAPE_SEQUENCE_STATE = 4;
+    private const WHITESPACE_STATE = 5;
+    private const COMMENT_STATE = 6;
+    private const REJECT_STATES = [self::SINGLE_QUOTED_STATE, self::DOUBLE_QUOTED_STATE, self::ESCAPE_SEQUENCE_STATE];
+
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Parse a raw entry into a proper entry.
+     *
+     * That is, turn a raw environment variable entry into a name and possibly
+     * a value. We wrap the answer in a result type.
+     *
+     * @param string $entry
+     *
+     * @return \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Entry,string>
+     */
+    public static function parse(string $entry)
+    {
+        return self::splitStringIntoParts($entry)->flatMap(static function (array $parts) {
+            [$name, $value] = $parts;
+
+            return self::parseName($name)->flatMap(static function (string $name) use ($value) {
+                /** @var Result<Value|null,string> */
+                $parsedValue = $value === null ? Success::create(null) : self::parseValue($value);
+
+                return $parsedValue->map(static function (?Value $value) use ($name) {
+                    return new Entry($name, $value);
+                });
+            });
+        });
+    }
+
+    /**
+     * Split the compound string into parts.
+     *
+     * @param string $line
+     *
+     * @return \GrahamCampbell\ResultType\Result<array{string,string|null},string>
+     */
+    private static function splitStringIntoParts(string $line)
+    {
+        /** @var array{string,string|null} */
+        $result = Str::pos($line, '=')->map(static function () use ($line) {
+            return \array_map('trim', \explode('=', $line, 2));
+        })->getOrElse([$line, null]);
+
+        if ($result[0] === '') {
+            /** @var \GrahamCampbell\ResultType\Result<array{string,string|null},string> */
+            return Error::create(self::getErrorMessage('an unexpected equals', $line));
+        }
+
+        /** @var \GrahamCampbell\ResultType\Result<array{string,string|null},string> */
+        return Success::create($result);
+    }
+
+    /**
+     * Parse the given variable name.
+     *
+     * That is, strip the optional quotes and leading "export" from the
+     * variable name. We wrap the answer in a result type.
+     *
+     * @param string $name
+     *
+     * @return \GrahamCampbell\ResultType\Result<string,string>
+     */
+    private static function parseName(string $name)
+    {
+        if (Str::len($name) > 8 && Str::substr($name, 0, 6) === 'export' && \ctype_space(Str::substr($name, 6, 1))) {
+            $name = \ltrim(Str::substr($name, 6));
+        }
+
+        if (self::isQuotedName($name)) {
+            $name = Str::substr($name, 1, -1);
+        }
+
+        if (!self::isValidName($name)) {
+            /** @var \GrahamCampbell\ResultType\Result<string,string> */
+            return Error::create(self::getErrorMessage('an invalid name', $name));
+        }
+
+        /** @var \GrahamCampbell\ResultType\Result<string,string> */
+        return Success::create($name);
+    }
+
+    /**
+     * Is the given variable name quoted?
+     *
+     * @param string $name
+     *
+     * @return bool
+     */
+    private static function isQuotedName(string $name)
+    {
+        if (Str::len($name) < 3) {
+            return false;
+        }
+
+        $first = Str::substr($name, 0, 1);
+        $last = Str::substr($name, -1, 1);
+
+        return ($first === '"' && $last === '"') || ($first === '\'' && $last === '\'');
+    }
+
+    /**
+     * Is the given variable name valid?
+     *
+     * @param string $name
+     *
+     * @return bool
+     */
+    private static function isValidName(string $name)
+    {
+        return Regex::matches('~(*UTF8)\A[\p{Ll}\p{Lu}\p{M}\p{N}_.]+\z~', $name)->success()->getOrElse(false);
+    }
+
+    /**
+     * Parse the given variable value.
+     *
+     * This has the effect of stripping quotes and comments, dealing with
+     * special characters, and locating nested variables, but not resolving
+     * them. Formally, we run a finite state automaton with an output tape: a
+     * transducer. We wrap the answer in a result type.
+     *
+     * @param string $value
+     *
+     * @return \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Value,string>
+     */
+    private static function parseValue(string $value)
+    {
+        if (\trim($value) === '') {
+            /** @var \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Value,string> */
+            return Success::create(Value::blank());
+        }
+
+        return \array_reduce(\iterator_to_array(Lexer::lex($value)), static function (Result $data, string $token) {
+            return $data->flatMap(static function (array $data) use ($token) {
+                return self::processToken($data[1], $token)->map(static function (array $val) use ($data) {
+                    return [$data[0]->append($val[0], $val[1]), $val[2]];
+                });
+            });
+        }, Success::create([Value::blank(), self::INITIAL_STATE]))->flatMap(static function (array $result) {
+            /** @psalm-suppress DocblockTypeContradiction */
+            if (in_array($result[1], self::REJECT_STATES, true)) {
+                /** @var \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Value,string> */
+                return Error::create('a missing closing quote');
+            }
+
+            /** @var \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Value,string> */
+            return Success::create($result[0]);
+        })->mapError(static function (string $err) use ($value) {
+            return self::getErrorMessage($err, $value);
+        });
+    }
+
+    /**
+     * Process the given token.
+     *
+     * @param int    $state
+     * @param string $token
+     *
+     * @return \GrahamCampbell\ResultType\Result<array{string,bool,int},string>
+     */
+    private static function processToken(int $state, string $token)
+    {
+        switch ($state) {
+            case self::INITIAL_STATE:
+                if ($token === '\'') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::SINGLE_QUOTED_STATE]);
+                } elseif ($token === '"') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::DOUBLE_QUOTED_STATE]);
+                } elseif ($token === '#') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::COMMENT_STATE]);
+                } elseif ($token === '$') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, true, self::UNQUOTED_STATE]);
+                } else {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, false, self::UNQUOTED_STATE]);
+                }
+            case self::UNQUOTED_STATE:
+                if ($token === '#') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::COMMENT_STATE]);
+                } elseif (\ctype_space($token)) {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::WHITESPACE_STATE]);
+                } elseif ($token === '$') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, true, self::UNQUOTED_STATE]);
+                } else {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, false, self::UNQUOTED_STATE]);
+                }
+            case self::SINGLE_QUOTED_STATE:
+                if ($token === '\'') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::WHITESPACE_STATE]);
+                } else {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, false, self::SINGLE_QUOTED_STATE]);
+                }
+            case self::DOUBLE_QUOTED_STATE:
+                if ($token === '"') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::WHITESPACE_STATE]);
+                } elseif ($token === '\\') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::ESCAPE_SEQUENCE_STATE]);
+                } elseif ($token === '$') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, true, self::DOUBLE_QUOTED_STATE]);
+                } else {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, false, self::DOUBLE_QUOTED_STATE]);
+                }
+            case self::ESCAPE_SEQUENCE_STATE:
+                if ($token === '"' || $token === '\\') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, false, self::DOUBLE_QUOTED_STATE]);
+                } elseif ($token === '$') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create([$token, false, self::DOUBLE_QUOTED_STATE]);
+                } else {
+                    $first = Str::substr($token, 0, 1);
+                    if (\in_array($first, ['f', 'n', 'r', 't', 'v'], true)) {
+                        /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                        return Success::create([\stripcslashes('\\'.$first).Str::substr($token, 1), false, self::DOUBLE_QUOTED_STATE]);
+                    } else {
+                        /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                        return Error::create('an unexpected escape sequence');
+                    }
+                }
+            case self::WHITESPACE_STATE:
+                if ($token === '#') {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::COMMENT_STATE]);
+                } elseif (!\ctype_space($token)) {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Error::create('unexpected whitespace');
+                } else {
+                    /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                    return Success::create(['', false, self::WHITESPACE_STATE]);
+                }
+            case self::COMMENT_STATE:
+                /** @var \GrahamCampbell\ResultType\Result<array{string,bool,int},string> */
+                return Success::create(['', false, self::COMMENT_STATE]);
+            default:
+                throw new \Error('Parser entered invalid state.');
+        }
+    }
+
+    /**
+     * Generate a friendly error message.
+     *
+     * @param string $cause
+     * @param string $subject
+     *
+     * @return string
+     */
+    private static function getErrorMessage(string $cause, string $subject)
+    {
+        return \sprintf(
+            'Encountered %s at [%s].',
+            $cause,
+            \strtok($subject, "\n")
+        );
+    }
+}
diff --git a/src/Dotenv/Parser/Lexer.php b/src/Dotenv/Parser/Lexer.php
new file mode 100644
index 00000000..981af24f
--- /dev/null
+++ b/src/Dotenv/Parser/Lexer.php
@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+final class Lexer
+{
+    /**
+     * The regex for each type of token.
+     */
+    private const PATTERNS = [
+        '[\r\n]{1,1000}', '[^\S\r\n]{1,1000}', '\\\\', '\'', '"', '\\#', '\\$', '([^(\s\\\\\'"\\#\\$)]|\\(|\\)){1,1000}',
+    ];
+
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Convert content into a token stream.
+     *
+     * Multibyte string processing is not needed here, and nether is error
+     * handling, for performance reasons.
+     *
+     * @param string $content
+     *
+     * @return \Generator<string>
+     */
+    public static function lex(string $content)
+    {
+        static $regex;
+
+        if ($regex === null) {
+            $regex = '(('.\implode(')|(', self::PATTERNS).'))A';
+        }
+
+        $offset = 0;
+
+        while (isset($content[$offset])) {
+            if (!\preg_match($regex, $content, $matches, 0, $offset)) {
+                throw new \Error(\sprintf('Lexer encountered unexpected character [%s].', $content[$offset]));
+            }
+
+            $offset += \strlen($matches[0]);
+
+            yield $matches[0];
+        }
+    }
+}
diff --git a/src/Dotenv/Parser/Lines.php b/src/Dotenv/Parser/Lines.php
new file mode 100644
index 00000000..64979932
--- /dev/null
+++ b/src/Dotenv/Parser/Lines.php
@@ -0,0 +1,127 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+use Dotenv\Util\Regex;
+use Dotenv\Util\Str;
+
+final class Lines
+{
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Process the array of lines of environment variables.
+     *
+     * This will produce an array of raw entries, one per variable.
+     *
+     * @param string[] $lines
+     *
+     * @return string[]
+     */
+    public static function process(array $lines)
+    {
+        $output = [];
+        $multiline = false;
+        $multilineBuffer = [];
+
+        foreach ($lines as $line) {
+            [$multiline, $line, $multilineBuffer] = self::multilineProcess($multiline, $line, $multilineBuffer);
+
+            if (!$multiline && !self::isCommentOrWhitespace($line)) {
+                $output[] = $line;
+            }
+        }
+
+        return $output;
+    }
+
+    /**
+     * Used to make all multiline variable process.
+     *
+     * @param bool     $multiline
+     * @param string   $line
+     * @param string[] $buffer
+     *
+     * @return array{bool,string,string[]}
+     */
+    private static function multilineProcess(bool $multiline, string $line, array $buffer)
+    {
+        $startsOnCurrentLine = $multiline ? false : self::looksLikeMultilineStart($line);
+
+        // check if $line can be multiline variable
+        if ($startsOnCurrentLine) {
+            $multiline = true;
+        }
+
+        if ($multiline) {
+            \array_push($buffer, $line);
+
+            if (self::looksLikeMultilineStop($line, $startsOnCurrentLine)) {
+                $multiline = false;
+                $line = \implode("\n", $buffer);
+                $buffer = [];
+            }
+        }
+
+        return [$multiline, $line, $buffer];
+    }
+
+    /**
+     * Determine if the given line can be the start of a multiline variable.
+     *
+     * @param string $line
+     *
+     * @return bool
+     */
+    private static function looksLikeMultilineStart(string $line)
+    {
+        return Str::pos($line, '="')->map(static function () use ($line) {
+            return self::looksLikeMultilineStop($line, true) === false;
+        })->getOrElse(false);
+    }
+
+    /**
+     * Determine if the given line can be the start of a multiline variable.
+     *
+     * @param string $line
+     * @param bool   $started
+     *
+     * @return bool
+     */
+    private static function looksLikeMultilineStop(string $line, bool $started)
+    {
+        if ($line === '"') {
+            return true;
+        }
+
+        return Regex::occurrences('/(?=([^\\\\]"))/', \str_replace('\\\\', '', $line))->map(static function (int $count) use ($started) {
+            return $started ? $count > 1 : $count >= 1;
+        })->success()->getOrElse(false);
+    }
+
+    /**
+     * Determine if the line in the file is a comment or whitespace.
+     *
+     * @param string $line
+     *
+     * @return bool
+     */
+    private static function isCommentOrWhitespace(string $line)
+    {
+        $line = \trim($line);
+
+        return $line === '' || (isset($line[0]) && $line[0] === '#');
+    }
+}
diff --git a/src/Dotenv/Parser/Parser.php b/src/Dotenv/Parser/Parser.php
new file mode 100644
index 00000000..2d30dfd6
--- /dev/null
+++ b/src/Dotenv/Parser/Parser.php
@@ -0,0 +1,53 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+use Dotenv\Exception\InvalidFileException;
+use Dotenv\Util\Regex;
+use GrahamCampbell\ResultType\Result;
+use GrahamCampbell\ResultType\Success;
+
+final class Parser implements ParserInterface
+{
+    /**
+     * Parse content into an entry array.
+     *
+     * @param string $content
+     *
+     * @throws \Dotenv\Exception\InvalidFileException
+     *
+     * @return \Dotenv\Parser\Entry[]
+     */
+    public function parse(string $content)
+    {
+        return Regex::split("/(\r\n|\n|\r)/", $content)->mapError(static function () {
+            return 'Could not split into separate lines.';
+        })->flatMap(static function (array $lines) {
+            return self::process(Lines::process($lines));
+        })->mapError(static function (string $error) {
+            throw new InvalidFileException(\sprintf('Failed to parse dotenv file. %s', $error));
+        })->success()->get();
+    }
+
+    /**
+     * Convert the raw entries into proper entries.
+     *
+     * @param string[] $entries
+     *
+     * @return \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Entry[],string>
+     */
+    private static function process(array $entries)
+    {
+        /** @var \GrahamCampbell\ResultType\Result<\Dotenv\Parser\Entry[],string> */
+        return \array_reduce($entries, static function (Result $result, string $raw) {
+            return $result->flatMap(static function (array $entries) use ($raw) {
+                return EntryParser::parse($raw)->map(static function (Entry $entry) use ($entries) {
+                    /** @var \Dotenv\Parser\Entry[] */
+                    return \array_merge($entries, [$entry]);
+                });
+            });
+        }, Success::create([]));
+    }
+}
diff --git a/src/Dotenv/Parser/ParserInterface.php b/src/Dotenv/Parser/ParserInterface.php
new file mode 100644
index 00000000..17cc42ad
--- /dev/null
+++ b/src/Dotenv/Parser/ParserInterface.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+interface ParserInterface
+{
+    /**
+     * Parse content into an entry array.
+     *
+     * @param string $content
+     *
+     * @throws \Dotenv\Exception\InvalidFileException
+     *
+     * @return \Dotenv\Parser\Entry[]
+     */
+    public function parse(string $content);
+}
diff --git a/src/Dotenv/Parser/Value.php b/src/Dotenv/Parser/Value.php
new file mode 100644
index 00000000..9e495a1e
--- /dev/null
+++ b/src/Dotenv/Parser/Value.php
@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Parser;
+
+use Dotenv\Util\Str;
+
+final class Value
+{
+    /**
+     * The string representation of the parsed value.
+     *
+     * @var string
+     */
+    private $chars;
+
+    /**
+     * The locations of the variables in the value.
+     *
+     * @var int[]
+     */
+    private $vars;
+
+    /**
+     * Internal constructor for a value.
+     *
+     * @param string $chars
+     * @param int[]  $vars
+     *
+     * @return void
+     */
+    private function __construct(string $chars, array $vars)
+    {
+        $this->chars = $chars;
+        $this->vars = $vars;
+    }
+
+    /**
+     * Create an empty value instance.
+     *
+     * @return \Dotenv\Parser\Value
+     */
+    public static function blank()
+    {
+        return new self('', []);
+    }
+
+    /**
+     * Create a new value instance, appending the characters.
+     *
+     * @param string $chars
+     * @param bool   $var
+     *
+     * @return \Dotenv\Parser\Value
+     */
+    public function append(string $chars, bool $var)
+    {
+        return new self(
+            $this->chars.$chars,
+            $var ? \array_merge($this->vars, [Str::len($this->chars)]) : $this->vars
+        );
+    }
+
+    /**
+     * Get the string representation of the parsed value.
+     *
+     * @return string
+     */
+    public function getChars()
+    {
+        return $this->chars;
+    }
+
+    /**
+     * Get the locations of the variables in the value.
+     *
+     * @return int[]
+     */
+    public function getVars()
+    {
+        $vars = $this->vars;
+
+        \rsort($vars);
+
+        return $vars;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/AdapterInterface.php b/src/Dotenv/Repository/Adapter/AdapterInterface.php
new file mode 100644
index 00000000..5604398a
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/AdapterInterface.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+interface AdapterInterface extends ReaderInterface, WriterInterface
+{
+    /**
+     * Create a new instance of the adapter, if it is available.
+     *
+     * @return \PhpOption\Option<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    public static function create();
+}
diff --git a/src/Dotenv/Repository/Adapter/ApacheAdapter.php b/src/Dotenv/Repository/Adapter/ApacheAdapter.php
new file mode 100644
index 00000000..af0aae11
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/ApacheAdapter.php
@@ -0,0 +1,89 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+use PhpOption\None;
+use PhpOption\Option;
+use PhpOption\Some;
+
+final class ApacheAdapter implements AdapterInterface
+{
+    /**
+     * Create a new apache adapter instance.
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Create a new instance of the adapter, if it is available.
+     *
+     * @return \PhpOption\Option<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    public static function create()
+    {
+        if (self::isSupported()) {
+            /** @var \PhpOption\Option<AdapterInterface> */
+            return Some::create(new self());
+        }
+
+        return None::create();
+    }
+
+    /**
+     * Determines if the adapter is supported.
+     *
+     * This happens if PHP is running as an Apache module.
+     *
+     * @return bool
+     */
+    private static function isSupported()
+    {
+        return \function_exists('apache_getenv') && \function_exists('apache_setenv');
+    }
+
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name)
+    {
+        /** @var \PhpOption\Option<string> */
+        return Option::fromValue(apache_getenv($name))->filter(static function ($value) {
+            return \is_string($value) && $value !== '';
+        });
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        return apache_setenv($name, $value);
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        return apache_setenv($name, '');
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/ArrayAdapter.php b/src/Dotenv/Repository/Adapter/ArrayAdapter.php
new file mode 100644
index 00000000..df64cf6d
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/ArrayAdapter.php
@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+use PhpOption\Option;
+use PhpOption\Some;
+
+final class ArrayAdapter implements AdapterInterface
+{
+    /**
+     * The variables and their values.
+     *
+     * @var array<string,string>
+     */
+    private $variables;
+
+    /**
+     * Create a new array adapter instance.
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        $this->variables = [];
+    }
+
+    /**
+     * Create a new instance of the adapter, if it is available.
+     *
+     * @return \PhpOption\Option<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    public static function create()
+    {
+        /** @var \PhpOption\Option<AdapterInterface> */
+        return Some::create(new self());
+    }
+
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name)
+    {
+        return Option::fromArraysValue($this->variables, $name);
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        $this->variables[$name] = $value;
+
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        unset($this->variables[$name]);
+
+        return true;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/EnvConstAdapter.php b/src/Dotenv/Repository/Adapter/EnvConstAdapter.php
new file mode 100644
index 00000000..9eb19477
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/EnvConstAdapter.php
@@ -0,0 +1,89 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+use PhpOption\Option;
+use PhpOption\Some;
+
+final class EnvConstAdapter implements AdapterInterface
+{
+    /**
+     * Create a new env const adapter instance.
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Create a new instance of the adapter, if it is available.
+     *
+     * @return \PhpOption\Option<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    public static function create()
+    {
+        /** @var \PhpOption\Option<AdapterInterface> */
+        return Some::create(new self());
+    }
+
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name)
+    {
+        /** @var \PhpOption\Option<string> */
+        return Option::fromArraysValue($_ENV, $name)
+            ->filter(static function ($value) {
+                return \is_scalar($value);
+            })
+            ->map(static function ($value) {
+                if ($value === false) {
+                    return 'false';
+                }
+
+                if ($value === true) {
+                    return 'true';
+                }
+
+                /** @psalm-suppress PossiblyInvalidCast */
+                return (string) $value;
+            });
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        $_ENV[$name] = $value;
+
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        unset($_ENV[$name]);
+
+        return true;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/GuardedWriter.php b/src/Dotenv/Repository/Adapter/GuardedWriter.php
new file mode 100644
index 00000000..fed8b9ba
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/GuardedWriter.php
@@ -0,0 +1,85 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+final class GuardedWriter implements WriterInterface
+{
+    /**
+     * The inner writer to use.
+     *
+     * @var \Dotenv\Repository\Adapter\WriterInterface
+     */
+    private $writer;
+
+    /**
+     * The variable name allow list.
+     *
+     * @var string[]
+     */
+    private $allowList;
+
+    /**
+     * Create a new guarded writer instance.
+     *
+     * @param \Dotenv\Repository\Adapter\WriterInterface $writer
+     * @param string[]                                   $allowList
+     *
+     * @return void
+     */
+    public function __construct(WriterInterface $writer, array $allowList)
+    {
+        $this->writer = $writer;
+        $this->allowList = $allowList;
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        // Don't set non-allowed variables
+        if (!$this->isAllowed($name)) {
+            return false;
+        }
+
+        // Set the value on the inner writer
+        return $this->writer->write($name, $value);
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        // Don't clear non-allowed variables
+        if (!$this->isAllowed($name)) {
+            return false;
+        }
+
+        // Set the value on the inner writer
+        return $this->writer->delete($name);
+    }
+
+    /**
+     * Determine if the given variable is allowed.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    private function isAllowed(string $name)
+    {
+        return \in_array($name, $this->allowList, true);
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/ImmutableWriter.php b/src/Dotenv/Repository/Adapter/ImmutableWriter.php
new file mode 100644
index 00000000..399e6f9b
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/ImmutableWriter.php
@@ -0,0 +1,110 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+final class ImmutableWriter implements WriterInterface
+{
+    /**
+     * The inner writer to use.
+     *
+     * @var \Dotenv\Repository\Adapter\WriterInterface
+     */
+    private $writer;
+
+    /**
+     * The inner reader to use.
+     *
+     * @var \Dotenv\Repository\Adapter\ReaderInterface
+     */
+    private $reader;
+
+    /**
+     * The record of loaded variables.
+     *
+     * @var array<string,string>
+     */
+    private $loaded;
+
+    /**
+     * Create a new immutable writer instance.
+     *
+     * @param \Dotenv\Repository\Adapter\WriterInterface $writer
+     * @param \Dotenv\Repository\Adapter\ReaderInterface $reader
+     *
+     * @return void
+     */
+    public function __construct(WriterInterface $writer, ReaderInterface $reader)
+    {
+        $this->writer = $writer;
+        $this->reader = $reader;
+        $this->loaded = [];
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        // Don't overwrite existing environment variables
+        // Ruby's dotenv does this with `ENV[key] ||= value`
+        if ($this->isExternallyDefined($name)) {
+            return false;
+        }
+
+        // Set the value on the inner writer
+        if (!$this->writer->write($name, $value)) {
+            return false;
+        }
+
+        // Record that we have loaded the variable
+        $this->loaded[$name] = '';
+
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        // Don't clear existing environment variables
+        if ($this->isExternallyDefined($name)) {
+            return false;
+        }
+
+        // Clear the value on the inner writer
+        if (!$this->writer->delete($name)) {
+            return false;
+        }
+
+        // Leave the variable as fair game
+        unset($this->loaded[$name]);
+
+        return true;
+    }
+
+    /**
+     * Determine if the given variable is externally defined.
+     *
+     * That is, is it an "existing" variable.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    private function isExternallyDefined(string $name)
+    {
+        return $this->reader->read($name)->isDefined() && !isset($this->loaded[$name]);
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/MultiReader.php b/src/Dotenv/Repository/Adapter/MultiReader.php
new file mode 100644
index 00000000..0cfda6f6
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/MultiReader.php
@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+use PhpOption\None;
+
+final class MultiReader implements ReaderInterface
+{
+    /**
+     * The set of readers to use.
+     *
+     * @var \Dotenv\Repository\Adapter\ReaderInterface[]
+     */
+    private $readers;
+
+    /**
+     * Create a new multi-reader instance.
+     *
+     * @param \Dotenv\Repository\Adapter\ReaderInterface[] $readers
+     *
+     * @return void
+     */
+    public function __construct(array $readers)
+    {
+        $this->readers = $readers;
+    }
+
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name)
+    {
+        foreach ($this->readers as $reader) {
+            $result = $reader->read($name);
+            if ($result->isDefined()) {
+                return $result;
+            }
+        }
+
+        return None::create();
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/MultiWriter.php b/src/Dotenv/Repository/Adapter/MultiWriter.php
new file mode 100644
index 00000000..15a9d8fd
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/MultiWriter.php
@@ -0,0 +1,64 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+final class MultiWriter implements WriterInterface
+{
+    /**
+     * The set of writers to use.
+     *
+     * @var \Dotenv\Repository\Adapter\WriterInterface[]
+     */
+    private $writers;
+
+    /**
+     * Create a new multi-writer instance.
+     *
+     * @param \Dotenv\Repository\Adapter\WriterInterface[] $writers
+     *
+     * @return void
+     */
+    public function __construct(array $writers)
+    {
+        $this->writers = $writers;
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        foreach ($this->writers as $writers) {
+            if (!$writers->write($name, $value)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        foreach ($this->writers as $writers) {
+            if (!$writers->delete($name)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/PutenvAdapter.php b/src/Dotenv/Repository/Adapter/PutenvAdapter.php
new file mode 100644
index 00000000..6d017cdb
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/PutenvAdapter.php
@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+use PhpOption\None;
+use PhpOption\Option;
+use PhpOption\Some;
+
+final class PutenvAdapter implements AdapterInterface
+{
+    /**
+     * Create a new putenv adapter instance.
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Create a new instance of the adapter, if it is available.
+     *
+     * @return \PhpOption\Option<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    public static function create()
+    {
+        if (self::isSupported()) {
+            /** @var \PhpOption\Option<AdapterInterface> */
+            return Some::create(new self());
+        }
+
+        return None::create();
+    }
+
+    /**
+     * Determines if the adapter is supported.
+     *
+     * @return bool
+     */
+    private static function isSupported()
+    {
+        return \function_exists('getenv') && \function_exists('putenv');
+    }
+
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name)
+    {
+        /** @var \PhpOption\Option<string> */
+        return Option::fromValue(\getenv($name), false)->filter(static function ($value) {
+            return \is_string($value);
+        });
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        \putenv("$name=$value");
+
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        \putenv($name);
+
+        return true;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/ReaderInterface.php b/src/Dotenv/Repository/Adapter/ReaderInterface.php
new file mode 100644
index 00000000..306a63fc
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/ReaderInterface.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+interface ReaderInterface
+{
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name);
+}
diff --git a/src/Dotenv/Repository/Adapter/ReplacingWriter.php b/src/Dotenv/Repository/Adapter/ReplacingWriter.php
new file mode 100644
index 00000000..98c0f041
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/ReplacingWriter.php
@@ -0,0 +1,104 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+final class ReplacingWriter implements WriterInterface
+{
+    /**
+     * The inner writer to use.
+     *
+     * @var \Dotenv\Repository\Adapter\WriterInterface
+     */
+    private $writer;
+
+    /**
+     * The inner reader to use.
+     *
+     * @var \Dotenv\Repository\Adapter\ReaderInterface
+     */
+    private $reader;
+
+    /**
+     * The record of seen variables.
+     *
+     * @var array<string,string>
+     */
+    private $seen;
+
+    /**
+     * Create a new replacement writer instance.
+     *
+     * @param \Dotenv\Repository\Adapter\WriterInterface $writer
+     * @param \Dotenv\Repository\Adapter\ReaderInterface $reader
+     *
+     * @return void
+     */
+    public function __construct(WriterInterface $writer, ReaderInterface $reader)
+    {
+        $this->writer = $writer;
+        $this->reader = $reader;
+        $this->seen = [];
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        if ($this->exists($name)) {
+            return $this->writer->write($name, $value);
+        }
+
+        // succeed if nothing to do
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        if ($this->exists($name)) {
+            return $this->writer->delete($name);
+        }
+
+        // succeed if nothing to do
+        return true;
+    }
+
+    /**
+     * Does the given environment variable exist.
+     *
+     * Returns true if it currently exists, or existed at any point in the past
+     * that we are aware of.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    private function exists(string $name)
+    {
+        if (isset($this->seen[$name])) {
+            return true;
+        }
+
+        if ($this->reader->read($name)->isDefined()) {
+            $this->seen[$name] = '';
+
+            return true;
+        }
+
+        return false;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/ServerConstAdapter.php b/src/Dotenv/Repository/Adapter/ServerConstAdapter.php
new file mode 100644
index 00000000..f93b6e5e
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/ServerConstAdapter.php
@@ -0,0 +1,89 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+use PhpOption\Option;
+use PhpOption\Some;
+
+final class ServerConstAdapter implements AdapterInterface
+{
+    /**
+     * Create a new server const adapter instance.
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Create a new instance of the adapter, if it is available.
+     *
+     * @return \PhpOption\Option<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    public static function create()
+    {
+        /** @var \PhpOption\Option<AdapterInterface> */
+        return Some::create(new self());
+    }
+
+    /**
+     * Read an environment variable, if it exists.
+     *
+     * @param non-empty-string $name
+     *
+     * @return \PhpOption\Option<string>
+     */
+    public function read(string $name)
+    {
+        /** @var \PhpOption\Option<string> */
+        return Option::fromArraysValue($_SERVER, $name)
+            ->filter(static function ($value) {
+                return \is_scalar($value);
+            })
+            ->map(static function ($value) {
+                if ($value === false) {
+                    return 'false';
+                }
+
+                if ($value === true) {
+                    return 'true';
+                }
+
+                /** @psalm-suppress PossiblyInvalidCast */
+                return (string) $value;
+            });
+    }
+
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value)
+    {
+        $_SERVER[$name] = $value;
+
+        return true;
+    }
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name)
+    {
+        unset($_SERVER[$name]);
+
+        return true;
+    }
+}
diff --git a/src/Dotenv/Repository/Adapter/WriterInterface.php b/src/Dotenv/Repository/Adapter/WriterInterface.php
new file mode 100644
index 00000000..4cb3d61f
--- /dev/null
+++ b/src/Dotenv/Repository/Adapter/WriterInterface.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository\Adapter;
+
+interface WriterInterface
+{
+    /**
+     * Write to an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     * @param string           $value
+     *
+     * @return bool
+     */
+    public function write(string $name, string $value);
+
+    /**
+     * Delete an environment variable, if possible.
+     *
+     * @param non-empty-string $name
+     *
+     * @return bool
+     */
+    public function delete(string $name);
+}
diff --git a/src/Dotenv/Repository/AdapterRepository.php b/src/Dotenv/Repository/AdapterRepository.php
new file mode 100644
index 00000000..e4b8fb78
--- /dev/null
+++ b/src/Dotenv/Repository/AdapterRepository.php
@@ -0,0 +1,107 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository;
+
+use Dotenv\Repository\Adapter\ReaderInterface;
+use Dotenv\Repository\Adapter\WriterInterface;
+use InvalidArgumentException;
+
+final class AdapterRepository implements RepositoryInterface
+{
+    /**
+     * The reader to use.
+     *
+     * @var \Dotenv\Repository\Adapter\ReaderInterface
+     */
+    private $reader;
+
+    /**
+     * The writer to use.
+     *
+     * @var \Dotenv\Repository\Adapter\WriterInterface
+     */
+    private $writer;
+
+    /**
+     * Create a new adapter repository instance.
+     *
+     * @param \Dotenv\Repository\Adapter\ReaderInterface $reader
+     * @param \Dotenv\Repository\Adapter\WriterInterface $writer
+     *
+     * @return void
+     */
+    public function __construct(ReaderInterface $reader, WriterInterface $writer)
+    {
+        $this->reader = $reader;
+        $this->writer = $writer;
+    }
+
+    /**
+     * Determine if the given environment variable is defined.
+     *
+     * @param string $name
+     *
+     * @return bool
+     */
+    public function has(string $name)
+    {
+        return '' !== $name && $this->reader->read($name)->isDefined();
+    }
+
+    /**
+     * Get an environment variable.
+     *
+     * @param string $name
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return string|null
+     */
+    public function get(string $name)
+    {
+        if ('' === $name) {
+            throw new InvalidArgumentException('Expected name to be a non-empty string.');
+        }
+
+        return $this->reader->read($name)->getOrElse(null);
+    }
+
+    /**
+     * Set an environment variable.
+     *
+     * @param string $name
+     * @param string $value
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return bool
+     */
+    public function set(string $name, string $value)
+    {
+        if ('' === $name) {
+            throw new InvalidArgumentException('Expected name to be a non-empty string.');
+        }
+
+        return $this->writer->write($name, $value);
+    }
+
+    /**
+     * Clear an environment variable.
+     *
+     * @param string $name
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return bool
+     */
+    public function clear(string $name)
+    {
+        if ('' === $name) {
+            throw new InvalidArgumentException('Expected name to be a non-empty string.');
+        }
+
+        return $this->writer->delete($name);
+    }
+}
diff --git a/src/Dotenv/Repository/RepositoryBuilder.php b/src/Dotenv/Repository/RepositoryBuilder.php
new file mode 100644
index 00000000..a042f9a1
--- /dev/null
+++ b/src/Dotenv/Repository/RepositoryBuilder.php
@@ -0,0 +1,272 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository;
+
+use Dotenv\Repository\Adapter\AdapterInterface;
+use Dotenv\Repository\Adapter\EnvConstAdapter;
+use Dotenv\Repository\Adapter\GuardedWriter;
+use Dotenv\Repository\Adapter\ImmutableWriter;
+use Dotenv\Repository\Adapter\MultiReader;
+use Dotenv\Repository\Adapter\MultiWriter;
+use Dotenv\Repository\Adapter\ReaderInterface;
+use Dotenv\Repository\Adapter\ServerConstAdapter;
+use Dotenv\Repository\Adapter\WriterInterface;
+use InvalidArgumentException;
+use PhpOption\Some;
+use ReflectionClass;
+
+final class RepositoryBuilder
+{
+    /**
+     * The set of default adapters.
+     */
+    private const DEFAULT_ADAPTERS = [
+        ServerConstAdapter::class,
+        EnvConstAdapter::class,
+    ];
+
+    /**
+     * The set of readers to use.
+     *
+     * @var \Dotenv\Repository\Adapter\ReaderInterface[]
+     */
+    private $readers;
+
+    /**
+     * The set of writers to use.
+     *
+     * @var \Dotenv\Repository\Adapter\WriterInterface[]
+     */
+    private $writers;
+
+    /**
+     * Are we immutable?
+     *
+     * @var bool
+     */
+    private $immutable;
+
+    /**
+     * The variable name allow list.
+     *
+     * @var string[]|null
+     */
+    private $allowList;
+
+    /**
+     * Create a new repository builder instance.
+     *
+     * @param \Dotenv\Repository\Adapter\ReaderInterface[] $readers
+     * @param \Dotenv\Repository\Adapter\WriterInterface[] $writers
+     * @param bool                                         $immutable
+     * @param string[]|null                                $allowList
+     *
+     * @return void
+     */
+    private function __construct(array $readers = [], array $writers = [], bool $immutable = false, array $allowList = null)
+    {
+        $this->readers = $readers;
+        $this->writers = $writers;
+        $this->immutable = $immutable;
+        $this->allowList = $allowList;
+    }
+
+    /**
+     * Create a new repository builder instance with no adapters added.
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public static function createWithNoAdapters()
+    {
+        return new self();
+    }
+
+    /**
+     * Create a new repository builder instance with the default adapters added.
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public static function createWithDefaultAdapters()
+    {
+        $adapters = \iterator_to_array(self::defaultAdapters());
+
+        return new self($adapters, $adapters);
+    }
+
+    /**
+     * Return the array of default adapters.
+     *
+     * @return \Generator<\Dotenv\Repository\Adapter\AdapterInterface>
+     */
+    private static function defaultAdapters()
+    {
+        foreach (self::DEFAULT_ADAPTERS as $adapter) {
+            $instance = $adapter::create();
+            if ($instance->isDefined()) {
+                yield $instance->get();
+            }
+        }
+    }
+
+    /**
+     * Determine if the given name if of an adapterclass.
+     *
+     * @param string $name
+     *
+     * @return bool
+     */
+    private static function isAnAdapterClass(string $name)
+    {
+        if (!\class_exists($name)) {
+            return false;
+        }
+
+        return (new ReflectionClass($name))->implementsInterface(AdapterInterface::class);
+    }
+
+    /**
+     * Creates a repository builder with the given reader added.
+     *
+     * Accepts either a reader instance, or a class-string for an adapter. If
+     * the adapter is not supported, then we silently skip adding it.
+     *
+     * @param \Dotenv\Repository\Adapter\ReaderInterface|string $reader
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public function addReader($reader)
+    {
+        if (!(\is_string($reader) && self::isAnAdapterClass($reader)) && !($reader instanceof ReaderInterface)) {
+            throw new InvalidArgumentException(
+                \sprintf(
+                    'Expected either an instance of %s or a class-string implementing %s',
+                    ReaderInterface::class,
+                    AdapterInterface::class
+                )
+            );
+        }
+
+        $optional = Some::create($reader)->flatMap(static function ($reader) {
+            return \is_string($reader) ? $reader::create() : Some::create($reader);
+        });
+
+        $readers = \array_merge($this->readers, \iterator_to_array($optional));
+
+        return new self($readers, $this->writers, $this->immutable, $this->allowList);
+    }
+
+    /**
+     * Creates a repository builder with the given writer added.
+     *
+     * Accepts either a writer instance, or a class-string for an adapter. If
+     * the adapter is not supported, then we silently skip adding it.
+     *
+     * @param \Dotenv\Repository\Adapter\WriterInterface|string $writer
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public function addWriter($writer)
+    {
+        if (!(\is_string($writer) && self::isAnAdapterClass($writer)) && !($writer instanceof WriterInterface)) {
+            throw new InvalidArgumentException(
+                \sprintf(
+                    'Expected either an instance of %s or a class-string implementing %s',
+                    WriterInterface::class,
+                    AdapterInterface::class
+                )
+            );
+        }
+
+        $optional = Some::create($writer)->flatMap(static function ($writer) {
+            return \is_string($writer) ? $writer::create() : Some::create($writer);
+        });
+
+        $writers = \array_merge($this->writers, \iterator_to_array($optional));
+
+        return new self($this->readers, $writers, $this->immutable, $this->allowList);
+    }
+
+    /**
+     * Creates a repository builder with the given adapter added.
+     *
+     * Accepts either an adapter instance, or a class-string for an adapter. If
+     * the adapter is not supported, then we silently skip adding it. We will
+     * add the adapter as both a reader and a writer.
+     *
+     * @param \Dotenv\Repository\Adapter\WriterInterface|string $adapter
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public function addAdapter($adapter)
+    {
+        if (!(\is_string($adapter) && self::isAnAdapterClass($adapter)) && !($adapter instanceof AdapterInterface)) {
+            throw new InvalidArgumentException(
+                \sprintf(
+                    'Expected either an instance of %s or a class-string implementing %s',
+                    WriterInterface::class,
+                    AdapterInterface::class
+                )
+            );
+        }
+
+        $optional = Some::create($adapter)->flatMap(static function ($adapter) {
+            return \is_string($adapter) ? $adapter::create() : Some::create($adapter);
+        });
+
+        $readers = \array_merge($this->readers, \iterator_to_array($optional));
+        $writers = \array_merge($this->writers, \iterator_to_array($optional));
+
+        return new self($readers, $writers, $this->immutable, $this->allowList);
+    }
+
+    /**
+     * Creates a repository builder with mutability enabled.
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public function immutable()
+    {
+        return new self($this->readers, $this->writers, true, $this->allowList);
+    }
+
+    /**
+     * Creates a repository builder with the given allow list.
+     *
+     * @param string[]|null $allowList
+     *
+     * @return \Dotenv\Repository\RepositoryBuilder
+     */
+    public function allowList(array $allowList = null)
+    {
+        return new self($this->readers, $this->writers, $this->immutable, $allowList);
+    }
+
+    /**
+     * Creates a new repository instance.
+     *
+     * @return \Dotenv\Repository\RepositoryInterface
+     */
+    public function make()
+    {
+        $reader = new MultiReader($this->readers);
+        $writer = new MultiWriter($this->writers);
+
+        if ($this->immutable) {
+            $writer = new ImmutableWriter($writer, $reader);
+        }
+
+        if ($this->allowList !== null) {
+            $writer = new GuardedWriter($writer, $this->allowList);
+        }
+
+        return new AdapterRepository($reader, $writer);
+    }
+}
diff --git a/src/Dotenv/Repository/RepositoryInterface.php b/src/Dotenv/Repository/RepositoryInterface.php
new file mode 100644
index 00000000..d9b18a40
--- /dev/null
+++ b/src/Dotenv/Repository/RepositoryInterface.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Repository;
+
+interface RepositoryInterface
+{
+    /**
+     * Determine if the given environment variable is defined.
+     *
+     * @param string $name
+     *
+     * @return bool
+     */
+    public function has(string $name);
+
+    /**
+     * Get an environment variable.
+     *
+     * @param string $name
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return string|null
+     */
+    public function get(string $name);
+
+    /**
+     * Set an environment variable.
+     *
+     * @param string $name
+     * @param string $value
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return bool
+     */
+    public function set(string $name, string $value);
+
+    /**
+     * Clear an environment variable.
+     *
+     * @param string $name
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return bool
+     */
+    public function clear(string $name);
+}
diff --git a/src/Dotenv/Store/File/Paths.php b/src/Dotenv/Store/File/Paths.php
new file mode 100644
index 00000000..4f678a55
--- /dev/null
+++ b/src/Dotenv/Store/File/Paths.php
@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Store\File;
+
+/**
+ * @internal
+ */
+final class Paths
+{
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Returns the full paths to the files.
+     *
+     * @param string[] $paths
+     * @param string[] $names
+     *
+     * @return string[]
+     */
+    public static function filePaths(array $paths, array $names)
+    {
+        $files = [];
+
+        foreach ($paths as $path) {
+            foreach ($names as $name) {
+                $files[] = \rtrim($path, \DIRECTORY_SEPARATOR).\DIRECTORY_SEPARATOR.$name;
+            }
+        }
+
+        return $files;
+    }
+}
diff --git a/src/Dotenv/Store/File/Reader.php b/src/Dotenv/Store/File/Reader.php
new file mode 100644
index 00000000..bcbbf7ad
--- /dev/null
+++ b/src/Dotenv/Store/File/Reader.php
@@ -0,0 +1,81 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Store\File;
+
+use Dotenv\Exception\InvalidEncodingException;
+use Dotenv\Util\Str;
+use PhpOption\Option;
+
+/**
+ * @internal
+ */
+final class Reader
+{
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Read the file(s), and return their raw content.
+     *
+     * We provide the file path as the key, and its content as the value. If
+     * short circuit mode is enabled, then the returned array with have length
+     * at most one. File paths that couldn't be read are omitted entirely.
+     *
+     * @param string[]    $filePaths
+     * @param bool        $shortCircuit
+     * @param string|null $fileEncoding
+     *
+     * @throws \Dotenv\Exception\InvalidEncodingException
+     *
+     * @return array<string,string>
+     */
+    public static function read(array $filePaths, bool $shortCircuit = true, string $fileEncoding = null)
+    {
+        $output = [];
+
+        foreach ($filePaths as $filePath) {
+            $content = self::readFromFile($filePath, $fileEncoding);
+            if ($content->isDefined()) {
+                $output[$filePath] = $content->get();
+                if ($shortCircuit) {
+                    break;
+                }
+            }
+        }
+
+        return $output;
+    }
+
+    /**
+     * Read the given file.
+     *
+     * @param string      $path
+     * @param string|null $encoding
+     *
+     * @throws \Dotenv\Exception\InvalidEncodingException
+     *
+     * @return \PhpOption\Option<string>
+     */
+    private static function readFromFile(string $path, string $encoding = null)
+    {
+        /** @var Option<string> */
+        $content = Option::fromValue(@\file_get_contents($path), false);
+
+        return $content->flatMap(static function (string $content) use ($encoding) {
+            return Str::utf8($content, $encoding)->mapError(static function (string $error) {
+                throw new InvalidEncodingException($error);
+            })->success();
+        });
+    }
+}
diff --git a/src/Dotenv/Store/FileStore.php b/src/Dotenv/Store/FileStore.php
new file mode 100644
index 00000000..43f6135c
--- /dev/null
+++ b/src/Dotenv/Store/FileStore.php
@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Store;
+
+use Dotenv\Exception\InvalidPathException;
+use Dotenv\Store\File\Reader;
+
+final class FileStore implements StoreInterface
+{
+    /**
+     * The file paths.
+     *
+     * @var string[]
+     */
+    private $filePaths;
+
+    /**
+     * Should file loading short circuit?
+     *
+     * @var bool
+     */
+    private $shortCircuit;
+
+    /**
+     * The file encoding.
+     *
+     * @var string|null
+     */
+    private $fileEncoding;
+
+    /**
+     * Create a new file store instance.
+     *
+     * @param string[]    $filePaths
+     * @param bool        $shortCircuit
+     * @param string|null $fileEncoding
+     *
+     * @return void
+     */
+    public function __construct(array $filePaths, bool $shortCircuit, string $fileEncoding = null)
+    {
+        $this->filePaths = $filePaths;
+        $this->shortCircuit = $shortCircuit;
+        $this->fileEncoding = $fileEncoding;
+    }
+
+    /**
+     * Read the content of the environment file(s).
+     *
+     * @throws \Dotenv\Exception\InvalidEncodingException|\Dotenv\Exception\InvalidPathException
+     *
+     * @return string
+     */
+    public function read()
+    {
+        if ($this->filePaths === []) {
+            throw new InvalidPathException('At least one environment file path must be provided.');
+        }
+
+        $contents = Reader::read($this->filePaths, $this->shortCircuit, $this->fileEncoding);
+
+        if (\count($contents) > 0) {
+            return \implode("\n", $contents);
+        }
+
+        throw new InvalidPathException(
+            \sprintf('Unable to read any of the environment file(s) at [%s].', \implode(', ', $this->filePaths))
+        );
+    }
+}
diff --git a/src/Dotenv/Store/StoreBuilder.php b/src/Dotenv/Store/StoreBuilder.php
new file mode 100644
index 00000000..304117fc
--- /dev/null
+++ b/src/Dotenv/Store/StoreBuilder.php
@@ -0,0 +1,141 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Store;
+
+use Dotenv\Store\File\Paths;
+
+final class StoreBuilder
+{
+    /**
+     * The of default name.
+     */
+    private const DEFAULT_NAME = '.env';
+
+    /**
+     * The paths to search within.
+     *
+     * @var string[]
+     */
+    private $paths;
+
+    /**
+     * The file names to search for.
+     *
+     * @var string[]
+     */
+    private $names;
+
+    /**
+     * Should file loading short circuit?
+     *
+     * @var bool
+     */
+    private $shortCircuit;
+
+    /**
+     * The file encoding.
+     *
+     * @var string|null
+     */
+    private $fileEncoding;
+
+    /**
+     * Create a new store builder instance.
+     *
+     * @param string[]    $paths
+     * @param string[]    $names
+     * @param bool        $shortCircuit
+     * @param string|null $fileEncoding
+     *
+     * @return void
+     */
+    private function __construct(array $paths = [], array $names = [], bool $shortCircuit = false, string $fileEncoding = null)
+    {
+        $this->paths = $paths;
+        $this->names = $names;
+        $this->shortCircuit = $shortCircuit;
+        $this->fileEncoding = $fileEncoding;
+    }
+
+    /**
+     * Create a new store builder instance with no names.
+     *
+     * @return \Dotenv\Store\StoreBuilder
+     */
+    public static function createWithNoNames()
+    {
+        return new self();
+    }
+
+    /**
+     * Create a new store builder instance with the default name.
+     *
+     * @return \Dotenv\Store\StoreBuilder
+     */
+    public static function createWithDefaultName()
+    {
+        return new self([], [self::DEFAULT_NAME]);
+    }
+
+    /**
+     * Creates a store builder with the given path added.
+     *
+     * @param string $path
+     *
+     * @return \Dotenv\Store\StoreBuilder
+     */
+    public function addPath(string $path)
+    {
+        return new self(\array_merge($this->paths, [$path]), $this->names, $this->shortCircuit, $this->fileEncoding);
+    }
+
+    /**
+     * Creates a store builder with the given name added.
+     *
+     * @param string $name
+     *
+     * @return \Dotenv\Store\StoreBuilder
+     */
+    public function addName(string $name)
+    {
+        return new self($this->paths, \array_merge($this->names, [$name]), $this->shortCircuit, $this->fileEncoding);
+    }
+
+    /**
+     * Creates a store builder with short circuit mode enabled.
+     *
+     * @return \Dotenv\Store\StoreBuilder
+     */
+    public function shortCircuit()
+    {
+        return new self($this->paths, $this->names, true, $this->fileEncoding);
+    }
+
+    /**
+     * Creates a store builder with the specified file encoding.
+     *
+     * @param string|null $fileEncoding
+     *
+     * @return \Dotenv\Store\StoreBuilder
+     */
+    public function fileEncoding(string $fileEncoding = null)
+    {
+        return new self($this->paths, $this->names, $this->shortCircuit, $fileEncoding);
+    }
+
+    /**
+     * Creates a new store instance.
+     *
+     * @return \Dotenv\Store\StoreInterface
+     */
+    public function make()
+    {
+        return new FileStore(
+            Paths::filePaths($this->paths, $this->names),
+            $this->shortCircuit,
+            $this->fileEncoding
+        );
+    }
+}
diff --git a/src/Dotenv/Store/StoreInterface.php b/src/Dotenv/Store/StoreInterface.php
new file mode 100644
index 00000000..6f5b9862
--- /dev/null
+++ b/src/Dotenv/Store/StoreInterface.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Store;
+
+interface StoreInterface
+{
+    /**
+     * Read the content of the environment file(s).
+     *
+     * @throws \Dotenv\Exception\InvalidEncodingException|\Dotenv\Exception\InvalidPathException
+     *
+     * @return string
+     */
+    public function read();
+}
diff --git a/src/Dotenv/Store/StringStore.php b/src/Dotenv/Store/StringStore.php
new file mode 100644
index 00000000..3f862a71
--- /dev/null
+++ b/src/Dotenv/Store/StringStore.php
@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Store;
+
+final class StringStore implements StoreInterface
+{
+    /**
+     * The file content.
+     *
+     * @var string
+     */
+    private $content;
+
+    /**
+     * Create a new string store instance.
+     *
+     * @param string $content
+     *
+     * @return void
+     */
+    public function __construct(string $content)
+    {
+        $this->content = $content;
+    }
+
+    /**
+     * Read the content of the environment file(s).
+     *
+     * @return string
+     */
+    public function read()
+    {
+        return $this->content;
+    }
+}
diff --git a/src/Dotenv/Util/Regex.php b/src/Dotenv/Util/Regex.php
new file mode 100644
index 00000000..52c15780
--- /dev/null
+++ b/src/Dotenv/Util/Regex.php
@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Util;
+
+use GrahamCampbell\ResultType\Error;
+use GrahamCampbell\ResultType\Success;
+
+/**
+ * @internal
+ */
+final class Regex
+{
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Perform a preg match, wrapping up the result.
+     *
+     * @param string $pattern
+     * @param string $subject
+     *
+     * @return \GrahamCampbell\ResultType\Result<bool,string>
+     */
+    public static function matches(string $pattern, string $subject)
+    {
+        return self::pregAndWrap(static function (string $subject) use ($pattern) {
+            return @\preg_match($pattern, $subject) === 1;
+        }, $subject);
+    }
+
+    /**
+     * Perform a preg match all, wrapping up the result.
+     *
+     * @param string $pattern
+     * @param string $subject
+     *
+     * @return \GrahamCampbell\ResultType\Result<int,string>
+     */
+    public static function occurrences(string $pattern, string $subject)
+    {
+        return self::pregAndWrap(static function (string $subject) use ($pattern) {
+            return (int) @\preg_match_all($pattern, $subject);
+        }, $subject);
+    }
+
+    /**
+     * Perform a preg replace callback, wrapping up the result.
+     *
+     * @param string   $pattern
+     * @param callable $callback
+     * @param string   $subject
+     * @param int|null $limit
+     *
+     * @return \GrahamCampbell\ResultType\Result<string,string>
+     */
+    public static function replaceCallback(string $pattern, callable $callback, string $subject, int $limit = null)
+    {
+        return self::pregAndWrap(static function (string $subject) use ($pattern, $callback, $limit) {
+            return (string) @\preg_replace_callback($pattern, $callback, $subject, $limit ?? -1);
+        }, $subject);
+    }
+
+    /**
+     * Perform a preg split, wrapping up the result.
+     *
+     * @param string $pattern
+     * @param string $subject
+     *
+     * @return \GrahamCampbell\ResultType\Result<string[],string>
+     */
+    public static function split(string $pattern, string $subject)
+    {
+        return self::pregAndWrap(static function (string $subject) use ($pattern) {
+            /** @var string[] */
+            return (array) @\preg_split($pattern, $subject);
+        }, $subject);
+    }
+
+    /**
+     * Perform a preg operation, wrapping up the result.
+     *
+     * @template V
+     *
+     * @param callable(string):V $operation
+     * @param string             $subject
+     *
+     * @return \GrahamCampbell\ResultType\Result<V,string>
+     */
+    private static function pregAndWrap(callable $operation, string $subject)
+    {
+        $result = $operation($subject);
+
+        if (\preg_last_error() !== \PREG_NO_ERROR) {
+            /** @var \GrahamCampbell\ResultType\Result<V,string> */
+            return Error::create(\preg_last_error_msg());
+        }
+
+        /** @var \GrahamCampbell\ResultType\Result<V,string> */
+        return Success::create($result);
+    }
+}
diff --git a/src/Dotenv/Util/Str.php b/src/Dotenv/Util/Str.php
new file mode 100644
index 00000000..087e236a
--- /dev/null
+++ b/src/Dotenv/Util/Str.php
@@ -0,0 +1,98 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv\Util;
+
+use GrahamCampbell\ResultType\Error;
+use GrahamCampbell\ResultType\Success;
+use PhpOption\Option;
+
+/**
+ * @internal
+ */
+final class Str
+{
+    /**
+     * This class is a singleton.
+     *
+     * @codeCoverageIgnore
+     *
+     * @return void
+     */
+    private function __construct()
+    {
+        //
+    }
+
+    /**
+     * Convert a string to UTF-8 from the given encoding.
+     *
+     * @param string      $input
+     * @param string|null $encoding
+     *
+     * @return \GrahamCampbell\ResultType\Result<string,string>
+     */
+    public static function utf8(string $input, string $encoding = null)
+    {
+        if ($encoding !== null && !\in_array($encoding, \mb_list_encodings(), true)) {
+            /** @var \GrahamCampbell\ResultType\Result<string,string> */
+            return Error::create(
+                \sprintf('Illegal character encoding [%s] specified.', $encoding)
+            );
+        }
+        $converted = $encoding === null ?
+            @\mb_convert_encoding($input, 'UTF-8') :
+            @\mb_convert_encoding($input, 'UTF-8', $encoding);
+        /**
+         * this is for support UTF-8 with BOM encoding
+         * @see https://en.wikipedia.org/wiki/Byte_order_mark
+         * @see https://github.com/vlucas/phpdotenv/issues/500
+         */
+        if (\substr($converted, 0, 3) == "\xEF\xBB\xBF") {
+            $converted = \substr($converted, 3);
+        }
+        /** @var \GrahamCampbell\ResultType\Result<string,string> */
+        return Success::create($converted);
+    }
+
+    /**
+     * Search for a given substring of the input.
+     *
+     * @param string $haystack
+     * @param string $needle
+     *
+     * @return \PhpOption\Option<int>
+     */
+    public static function pos(string $haystack, string $needle)
+    {
+        /** @var \PhpOption\Option<int> */
+        return Option::fromValue(\mb_strpos($haystack, $needle, 0, 'UTF-8'), false);
+    }
+
+    /**
+     * Grab the specified substring of the input.
+     *
+     * @param string   $input
+     * @param int      $start
+     * @param int|null $length
+     *
+     * @return string
+     */
+    public static function substr(string $input, int $start, int $length = null)
+    {
+        return \mb_substr($input, $start, $length, 'UTF-8');
+    }
+
+    /**
+     * Compute the length of the given string.
+     *
+     * @param string $input
+     *
+     * @return int
+     */
+    public static function len(string $input)
+    {
+        return \mb_strlen($input, 'UTF-8');
+    }
+}
diff --git a/src/Dotenv/Validator.php b/src/Dotenv/Validator.php
new file mode 100644
index 00000000..0c04ab62
--- /dev/null
+++ b/src/Dotenv/Validator.php
@@ -0,0 +1,209 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dotenv;
+
+use Dotenv\Exception\ValidationException;
+use Dotenv\Repository\RepositoryInterface;
+use Dotenv\Util\Regex;
+use Dotenv\Util\Str;
+
+class Validator
+{
+    /**
+     * The environment repository instance.
+     *
+     * @var \Dotenv\Repository\RepositoryInterface
+     */
+    private $repository;
+
+    /**
+     * The variables to validate.
+     *
+     * @var string[]
+     */
+    private $variables;
+
+    /**
+     * Create a new validator instance.
+     *
+     * @param \Dotenv\Repository\RepositoryInterface $repository
+     * @param string[]                               $variables
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return void
+     */
+    public function __construct(RepositoryInterface $repository, array $variables)
+    {
+        $this->repository = $repository;
+        $this->variables = $variables;
+    }
+
+    /**
+     * Assert that each variable is present.
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function required()
+    {
+        return $this->assert(
+            static function (?string $value) {
+                return $value !== null;
+            },
+            'is missing'
+        );
+    }
+
+    /**
+     * Assert that each variable is not empty.
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function notEmpty()
+    {
+        return $this->assertNullable(
+            static function (string $value) {
+                return Str::len(\trim($value)) > 0;
+            },
+            'is empty'
+        );
+    }
+
+    /**
+     * Assert that each specified variable is an integer.
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function isInteger()
+    {
+        return $this->assertNullable(
+            static function (string $value) {
+                return \ctype_digit($value);
+            },
+            'is not an integer'
+        );
+    }
+
+    /**
+     * Assert that each specified variable is a boolean.
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function isBoolean()
+    {
+        return $this->assertNullable(
+            static function (string $value) {
+                if ($value === '') {
+                    return false;
+                }
+
+                return \filter_var($value, \FILTER_VALIDATE_BOOLEAN, \FILTER_NULL_ON_FAILURE) !== null;
+            },
+            'is not a boolean'
+        );
+    }
+
+    /**
+     * Assert that each variable is amongst the given choices.
+     *
+     * @param string[] $choices
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function allowedValues(array $choices)
+    {
+        return $this->assertNullable(
+            static function (string $value) use ($choices) {
+                return \in_array($value, $choices, true);
+            },
+            \sprintf('is not one of [%s]', \implode(', ', $choices))
+        );
+    }
+
+    /**
+     * Assert that each variable matches the given regular expression.
+     *
+     * @param string $regex
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function allowedRegexValues(string $regex)
+    {
+        return $this->assertNullable(
+            static function (string $value) use ($regex) {
+                return Regex::matches($regex, $value)->success()->getOrElse(false);
+            },
+            \sprintf('does not match "%s"', $regex)
+        );
+    }
+
+    /**
+     * Assert that the callback returns true for each variable.
+     *
+     * @param callable(?string):bool $callback
+     * @param string                 $message
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function assert(callable $callback, string $message)
+    {
+        $failing = [];
+
+        foreach ($this->variables as $variable) {
+            if ($callback($this->repository->get($variable)) === false) {
+                $failing[] = \sprintf('%s %s', $variable, $message);
+            }
+        }
+
+        if (\count($failing) > 0) {
+            throw new ValidationException(\sprintf(
+                'One or more environment variables failed assertions: %s.',
+                \implode(', ', $failing)
+            ));
+        }
+
+        return $this;
+    }
+
+    /**
+     * Assert that the callback returns true for each variable.
+     *
+     * Skip checking null variable values.
+     *
+     * @param callable(string):bool $callback
+     * @param string                $message
+     *
+     * @throws \Dotenv\Exception\ValidationException
+     *
+     * @return \Dotenv\Validator
+     */
+    public function assertNullable(callable $callback, string $message)
+    {
+        return $this->assert(
+            static function (?string $value) use ($callback) {
+                if ($value === null) {
+                    return true;
+                }
+
+                return $callback($value);
+            },
+            $message
+        );
+    }
+}
diff --git a/src/GrahamCampbell/ResultType/Error.php b/src/GrahamCampbell/ResultType/Error.php
new file mode 100644
index 00000000..2c37c3e2
--- /dev/null
+++ b/src/GrahamCampbell/ResultType/Error.php
@@ -0,0 +1,121 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of Result Type.
+ *
+ * (c) Graham Campbell <hello@gjcampbell.co.uk>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace GrahamCampbell\ResultType;
+
+use PhpOption\None;
+use PhpOption\Some;
+
+/**
+ * @template T
+ * @template E
+ *
+ * @extends \GrahamCampbell\ResultType\Result<T,E>
+ */
+final class Error extends Result
+{
+    /**
+     * @var E
+     */
+    private $value;
+
+    /**
+     * Internal constructor for an error value.
+     *
+     * @param E $value
+     *
+     * @return void
+     */
+    private function __construct($value)
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * Create a new error value.
+     *
+     * @template F
+     *
+     * @param F $value
+     *
+     * @return \GrahamCampbell\ResultType\Result<T,F>
+     */
+    public static function create($value)
+    {
+        return new self($value);
+    }
+
+    /**
+     * Get the success option value.
+     *
+     * @return \PhpOption\Option<T>
+     */
+    public function success()
+    {
+        return None::create();
+    }
+
+    /**
+     * Map over the success value.
+     *
+     * @template S
+     *
+     * @param callable(T):S $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,E>
+     */
+    public function map(callable $f)
+    {
+        return self::create($this->value);
+    }
+
+    /**
+     * Flat map over the success value.
+     *
+     * @template S
+     * @template F
+     *
+     * @param callable(T):\GrahamCampbell\ResultType\Result<S,F> $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,F>
+     */
+    public function flatMap(callable $f)
+    {
+        /** @var \GrahamCampbell\ResultType\Result<S,F> */
+        return self::create($this->value);
+    }
+
+    /**
+     * Get the error option value.
+     *
+     * @return \PhpOption\Option<E>
+     */
+    public function error()
+    {
+        return Some::create($this->value);
+    }
+
+    /**
+     * Map over the error value.
+     *
+     * @template F
+     *
+     * @param callable(E):F $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<T,F>
+     */
+    public function mapError(callable $f)
+    {
+        return self::create($f($this->value));
+    }
+}
diff --git a/src/GrahamCampbell/ResultType/Result.php b/src/GrahamCampbell/ResultType/Result.php
new file mode 100644
index 00000000..8c67bcdd
--- /dev/null
+++ b/src/GrahamCampbell/ResultType/Result.php
@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of Result Type.
+ *
+ * (c) Graham Campbell <hello@gjcampbell.co.uk>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace GrahamCampbell\ResultType;
+
+/**
+ * @template T
+ * @template E
+ */
+abstract class Result
+{
+    /**
+     * Get the success option value.
+     *
+     * @return \PhpOption\Option<T>
+     */
+    abstract public function success();
+
+    /**
+     * Map over the success value.
+     *
+     * @template S
+     *
+     * @param callable(T):S $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,E>
+     */
+    abstract public function map(callable $f);
+
+    /**
+     * Flat map over the success value.
+     *
+     * @template S
+     * @template F
+     *
+     * @param callable(T):\GrahamCampbell\ResultType\Result<S,F> $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,F>
+     */
+    abstract public function flatMap(callable $f);
+
+    /**
+     * Get the error option value.
+     *
+     * @return \PhpOption\Option<E>
+     */
+    abstract public function error();
+
+    /**
+     * Map over the error value.
+     *
+     * @template F
+     *
+     * @param callable(E):F $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<T,F>
+     */
+    abstract public function mapError(callable $f);
+}
diff --git a/src/GrahamCampbell/ResultType/Success.php b/src/GrahamCampbell/ResultType/Success.php
new file mode 100644
index 00000000..27cd85ee
--- /dev/null
+++ b/src/GrahamCampbell/ResultType/Success.php
@@ -0,0 +1,120 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of Result Type.
+ *
+ * (c) Graham Campbell <hello@gjcampbell.co.uk>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace GrahamCampbell\ResultType;
+
+use PhpOption\None;
+use PhpOption\Some;
+
+/**
+ * @template T
+ * @template E
+ *
+ * @extends \GrahamCampbell\ResultType\Result<T,E>
+ */
+final class Success extends Result
+{
+    /**
+     * @var T
+     */
+    private $value;
+
+    /**
+     * Internal constructor for a success value.
+     *
+     * @param T $value
+     *
+     * @return void
+     */
+    private function __construct($value)
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * Create a new error value.
+     *
+     * @template S
+     *
+     * @param S $value
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,E>
+     */
+    public static function create($value)
+    {
+        return new self($value);
+    }
+
+    /**
+     * Get the success option value.
+     *
+     * @return \PhpOption\Option<T>
+     */
+    public function success()
+    {
+        return Some::create($this->value);
+    }
+
+    /**
+     * Map over the success value.
+     *
+     * @template S
+     *
+     * @param callable(T):S $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,E>
+     */
+    public function map(callable $f)
+    {
+        return self::create($f($this->value));
+    }
+
+    /**
+     * Flat map over the success value.
+     *
+     * @template S
+     * @template F
+     *
+     * @param callable(T):\GrahamCampbell\ResultType\Result<S,F> $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<S,F>
+     */
+    public function flatMap(callable $f)
+    {
+        return $f($this->value);
+    }
+
+    /**
+     * Get the error option value.
+     *
+     * @return \PhpOption\Option<E>
+     */
+    public function error()
+    {
+        return None::create();
+    }
+
+    /**
+     * Map over the error value.
+     *
+     * @template F
+     *
+     * @param callable(E):F $f
+     *
+     * @return \GrahamCampbell\ResultType\Result<T,F>
+     */
+    public function mapError(callable $f)
+    {
+        return self::create($this->value);
+    }
+}
diff --git a/src/PhpOption/LazyOption.php b/src/PhpOption/LazyOption.php
new file mode 100644
index 00000000..9cb77c86
--- /dev/null
+++ b/src/PhpOption/LazyOption.php
@@ -0,0 +1,175 @@
+<?php
+
+/*
+ * Copyright 2012 Johannes M. Schmitt <schmittjoh@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace PhpOption;
+
+use Traversable;
+
+/**
+ * @template T
+ *
+ * @extends Option<T>
+ */
+final class LazyOption extends Option
+{
+    /** @var callable(mixed...):(Option<T>) */
+    private $callback;
+
+    /** @var array<int, mixed> */
+    private $arguments;
+
+    /** @var Option<T>|null */
+    private $option;
+
+    /**
+     * @template S
+     * @param callable(mixed...):(Option<S>) $callback
+     * @param array<int, mixed>              $arguments
+     *
+     * @return LazyOption<S>
+     */
+    public static function create($callback, array $arguments = []): self
+    {
+        return new self($callback, $arguments);
+    }
+
+    /**
+     * @param callable(mixed...):(Option<T>) $callback
+     * @param array<int, mixed>              $arguments
+     */
+    public function __construct($callback, array $arguments = [])
+    {
+        if (!is_callable($callback)) {
+            throw new \InvalidArgumentException('Invalid callback given');
+        }
+
+        $this->callback = $callback;
+        $this->arguments = $arguments;
+    }
+
+    public function isDefined(): bool
+    {
+        return $this->option()->isDefined();
+    }
+
+    public function isEmpty(): bool
+    {
+        return $this->option()->isEmpty();
+    }
+
+    public function get()
+    {
+        return $this->option()->get();
+    }
+
+    public function getOrElse($default)
+    {
+        return $this->option()->getOrElse($default);
+    }
+
+    public function getOrCall($callable)
+    {
+        return $this->option()->getOrCall($callable);
+    }
+
+    public function getOrThrow(\Exception $ex)
+    {
+        return $this->option()->getOrThrow($ex);
+    }
+
+    public function orElse(Option $else)
+    {
+        return $this->option()->orElse($else);
+    }
+
+    public function ifDefined($callable)
+    {
+        $this->option()->forAll($callable);
+    }
+
+    public function forAll($callable)
+    {
+        return $this->option()->forAll($callable);
+    }
+
+    public function map($callable)
+    {
+        return $this->option()->map($callable);
+    }
+
+    public function flatMap($callable)
+    {
+        return $this->option()->flatMap($callable);
+    }
+
+    public function filter($callable)
+    {
+        return $this->option()->filter($callable);
+    }
+
+    public function filterNot($callable)
+    {
+        return $this->option()->filterNot($callable);
+    }
+
+    public function select($value)
+    {
+        return $this->option()->select($value);
+    }
+
+    public function reject($value)
+    {
+        return $this->option()->reject($value);
+    }
+
+    /**
+     * @return Traversable<T>
+     */
+    public function getIterator(): Traversable
+    {
+        return $this->option()->getIterator();
+    }
+
+    public function foldLeft($initialValue, $callable)
+    {
+        return $this->option()->foldLeft($initialValue, $callable);
+    }
+
+    public function foldRight($initialValue, $callable)
+    {
+        return $this->option()->foldRight($initialValue, $callable);
+    }
+
+    /**
+     * @return Option<T>
+     */
+    private function option(): Option
+    {
+        if (null === $this->option) {
+            /** @var mixed */
+            $option = call_user_func_array($this->callback, $this->arguments);
+            if ($option instanceof Option) {
+                $this->option = $option;
+            } else {
+                throw new \RuntimeException(sprintf('Expected instance of %s', Option::class));
+            }
+        }
+
+        return $this->option;
+    }
+}
diff --git a/src/PhpOption/None.php b/src/PhpOption/None.php
new file mode 100644
index 00000000..4b85d22d
--- /dev/null
+++ b/src/PhpOption/None.php
@@ -0,0 +1,136 @@
+<?php
+
+/*
+ * Copyright 2012 Johannes M. Schmitt <schmittjoh@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace PhpOption;
+
+use EmptyIterator;
+
+/**
+ * @extends Option<mixed>
+ */
+final class None extends Option
+{
+    /** @var None|null */
+    private static $instance;
+
+    /**
+     * @return None
+     */
+    public static function create(): self
+    {
+        if (null === self::$instance) {
+            self::$instance = new self();
+        }
+
+        return self::$instance;
+    }
+
+    public function get()
+    {
+        throw new \RuntimeException('None has no value.');
+    }
+
+    public function getOrCall($callable)
+    {
+        return $callable();
+    }
+
+    public function getOrElse($default)
+    {
+        return $default;
+    }
+
+    public function getOrThrow(\Exception $ex)
+    {
+        throw $ex;
+    }
+
+    public function isEmpty(): bool
+    {
+        return true;
+    }
+
+    public function isDefined(): bool
+    {
+        return false;
+    }
+
+    public function orElse(Option $else)
+    {
+        return $else;
+    }
+
+    public function ifDefined($callable)
+    {
+        // Just do nothing in that case.
+    }
+
+    public function forAll($callable)
+    {
+        return $this;
+    }
+
+    public function map($callable)
+    {
+        return $this;
+    }
+
+    public function flatMap($callable)
+    {
+        return $this;
+    }
+
+    public function filter($callable)
+    {
+        return $this;
+    }
+
+    public function filterNot($callable)
+    {
+        return $this;
+    }
+
+    public function select($value)
+    {
+        return $this;
+    }
+
+    public function reject($value)
+    {
+        return $this;
+    }
+
+    public function getIterator(): EmptyIterator
+    {
+        return new EmptyIterator();
+    }
+
+    public function foldLeft($initialValue, $callable)
+    {
+        return $initialValue;
+    }
+
+    public function foldRight($initialValue, $callable)
+    {
+        return $initialValue;
+    }
+
+    private function __construct()
+    {
+    }
+}
diff --git a/src/PhpOption/Option.php b/src/PhpOption/Option.php
new file mode 100644
index 00000000..172924cf
--- /dev/null
+++ b/src/PhpOption/Option.php
@@ -0,0 +1,434 @@
+<?php
+
+/*
+ * Copyright 2012 Johannes M. Schmitt <schmittjoh@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace PhpOption;
+
+use ArrayAccess;
+use IteratorAggregate;
+
+/**
+ * @template T
+ *
+ * @implements IteratorAggregate<T>
+ */
+abstract class Option implements IteratorAggregate
+{
+    /**
+     * Creates an option given a return value.
+     *
+     * This is intended for consuming existing APIs and allows you to easily
+     * convert them to an option. By default, we treat ``null`` as the None
+     * case, and everything else as Some.
+     *
+     * @template S
+     *
+     * @param S $value     The actual return value.
+     * @param S $noneValue The value which should be considered "None"; null by
+     *                     default.
+     *
+     * @return Option<S>
+     */
+    public static function fromValue($value, $noneValue = null)
+    {
+        if ($value === $noneValue) {
+            return None::create();
+        }
+
+        return new Some($value);
+    }
+
+    /**
+     * Creates an option from an array's value.
+     *
+     * If the key does not exist in the array, the array is not actually an
+     * array, or the array's value at the given key is null, None is returned.
+     * Otherwise, Some is returned wrapping the value at the given key.
+     *
+     * @template S
+     *
+     * @param array<string|int,S>|ArrayAccess<string|int,S>|null $array A potential array or \ArrayAccess value.
+     * @param string                                             $key   The key to check.
+     *
+     * @return Option<S>
+     */
+    public static function fromArraysValue($array, $key)
+    {
+        if (!(is_array($array) || $array instanceof ArrayAccess) || !isset($array[$key])) {
+            return None::create();
+        }
+
+        return new Some($array[$key]);
+    }
+
+    /**
+     * Creates a lazy-option with the given callback.
+     *
+     * This is also a helper constructor for lazy-consuming existing APIs where
+     * the return value is not yet an option. By default, we treat ``null`` as
+     * None case, and everything else as Some.
+     *
+     * @template S
+     *
+     * @param callable $callback  The callback to evaluate.
+     * @param array    $arguments The arguments for the callback.
+     * @param S        $noneValue The value which should be considered "None";
+    *                             null by default.
+     *
+     * @return LazyOption<S>
+     */
+    public static function fromReturn($callback, array $arguments = [], $noneValue = null)
+    {
+        return new LazyOption(static function () use ($callback, $arguments, $noneValue) {
+            /** @var mixed */
+            $return = call_user_func_array($callback, $arguments);
+
+            if ($return === $noneValue) {
+                return None::create();
+            }
+
+            return new Some($return);
+        });
+    }
+
+    /**
+     * Option factory, which creates new option based on passed value.
+     *
+     * If value is already an option, it simply returns. If value is callable,
+     * LazyOption with passed callback created and returned. If Option
+     * returned from callback, it returns directly. On other case value passed
+     * to Option::fromValue() method.
+     *
+     * @template S
+     *
+     * @param Option<S>|callable|S $value
+     * @param S                    $noneValue Used when $value is mixed or
+     *                                        callable, for None-check.
+     *
+     * @return Option<S>|LazyOption<S>
+     */
+    public static function ensure($value, $noneValue = null)
+    {
+        if ($value instanceof self) {
+            return $value;
+        } elseif (is_callable($value)) {
+            return new LazyOption(static function () use ($value, $noneValue) {
+                /** @var mixed */
+                $return = $value();
+
+                if ($return instanceof self) {
+                    return $return;
+                } else {
+                    return self::fromValue($return, $noneValue);
+                }
+            });
+        } else {
+            return self::fromValue($value, $noneValue);
+        }
+    }
+
+    /**
+     * Lift a function so that it accepts Option as parameters.
+     *
+     * We return a new closure that wraps the original callback. If any of the
+     * parameters passed to the lifted function is empty, the function will
+     * return a value of None. Otherwise, we will pass all parameters to the
+     * original callback and return the value inside a new Option, unless an
+     * Option is returned from the function, in which case, we use that.
+     *
+     * @template S
+     *
+     * @param callable $callback
+     * @param mixed    $noneValue
+     *
+     * @return callable
+     */
+    public static function lift($callback, $noneValue = null)
+    {
+        return static function () use ($callback, $noneValue) {
+            /** @var array<int, mixed> */
+            $args = func_get_args();
+
+            $reduced_args = array_reduce(
+                $args,
+                /** @param bool $status */
+                static function ($status, self $o) {
+                    return $o->isEmpty() ? true : $status;
+                },
+                false
+            );
+            // if at least one parameter is empty, return None
+            if ($reduced_args) {
+                return None::create();
+            }
+
+            $args = array_map(
+                /** @return T */
+                static function (self $o) {
+                    // it is safe to do so because the fold above checked
+                    // that all arguments are of type Some
+                    /** @var T */
+                    return $o->get();
+                },
+                $args
+            );
+
+            return self::ensure(call_user_func_array($callback, $args), $noneValue);
+        };
+    }
+
+    /**
+     * Returns the value if available, or throws an exception otherwise.
+     *
+     * @throws \RuntimeException If value is not available.
+     *
+     * @return T
+     */
+    abstract public function get();
+
+    /**
+     * Returns the value if available, or the default value if not.
+     *
+     * @template S
+     *
+     * @param S $default
+     *
+     * @return T|S
+     */
+    abstract public function getOrElse($default);
+
+    /**
+     * Returns the value if available, or the results of the callable.
+     *
+     * This is preferable over ``getOrElse`` if the computation of the default
+     * value is expensive.
+     *
+     * @template S
+     *
+     * @param callable():S $callable
+     *
+     * @return T|S
+     */
+    abstract public function getOrCall($callable);
+
+    /**
+     * Returns the value if available, or throws the passed exception.
+     *
+     * @param \Exception $ex
+     *
+     * @return T
+     */
+    abstract public function getOrThrow(\Exception $ex);
+
+    /**
+     * Returns true if no value is available, false otherwise.
+     *
+     * @return bool
+     */
+    abstract public function isEmpty();
+
+    /**
+     * Returns true if a value is available, false otherwise.
+     *
+     * @return bool
+     */
+    abstract public function isDefined();
+
+    /**
+     * Returns this option if non-empty, or the passed option otherwise.
+     *
+     * This can be used to try multiple alternatives, and is especially useful
+     * with lazy evaluating options:
+     *
+     * ```php
+     *     $repo->findSomething()
+     *         ->orElse(new LazyOption(array($repo, 'findSomethingElse')))
+     *         ->orElse(new LazyOption(array($repo, 'createSomething')));
+     * ```
+     *
+     * @param Option<T> $else
+     *
+     * @return Option<T>
+     */
+    abstract public function orElse(self $else);
+
+    /**
+     * This is similar to map() below except that the return value has no meaning;
+     * the passed callable is simply executed if the option is non-empty, and
+     * ignored if the option is empty.
+     *
+     * In all cases, the return value of the callable is discarded.
+     *
+     * ```php
+     *     $comment->getMaybeFile()->ifDefined(function($file) {
+     *         // Do something with $file here.
+     *     });
+     * ```
+     *
+     * If you're looking for something like ``ifEmpty``, you can use ``getOrCall``
+     * and ``getOrElse`` in these cases.
+     *
+     * @deprecated Use forAll() instead.
+     *
+     * @param callable(T):mixed $callable
+     *
+     * @return void
+     */
+    abstract public function ifDefined($callable);
+
+    /**
+     * This is similar to map() except that the return value of the callable has no meaning.
+     *
+     * The passed callable is simply executed if the option is non-empty, and ignored if the
+     * option is empty. This method is preferred for callables with side-effects, while map()
+     * is intended for callables without side-effects.
+     *
+     * @param callable(T):mixed $callable
+     *
+     * @return Option<T>
+     */
+    abstract public function forAll($callable);
+
+    /**
+     * Applies the callable to the value of the option if it is non-empty,
+     * and returns the return value of the callable wrapped in Some().
+     *
+     * If the option is empty, then the callable is not applied.
+     *
+     * ```php
+     *     (new Some("foo"))->map('strtoupper')->get(); // "FOO"
+     * ```
+     *
+     * @template S
+     *
+     * @param callable(T):S $callable
+     *
+     * @return Option<S>
+     */
+    abstract public function map($callable);
+
+    /**
+     * Applies the callable to the value of the option if it is non-empty, and
+     * returns the return value of the callable directly.
+     *
+     * In contrast to ``map``, the return value of the callable is expected to
+     * be an Option itself; it is not automatically wrapped in Some().
+     *
+     * @template S
+     *
+     * @param callable(T):Option<S> $callable must return an Option
+     *
+     * @return Option<S>
+     */
+    abstract public function flatMap($callable);
+
+    /**
+     * If the option is empty, it is returned immediately without applying the callable.
+     *
+     * If the option is non-empty, the callable is applied, and if it returns true,
+     * the option itself is returned; otherwise, None is returned.
+     *
+     * @param callable(T):bool $callable
+     *
+     * @return Option<T>
+     */
+    abstract public function filter($callable);
+
+    /**
+     * If the option is empty, it is returned immediately without applying the callable.
+     *
+     * If the option is non-empty, the callable is applied, and if it returns false,
+     * the option itself is returned; otherwise, None is returned.
+     *
+     * @param callable(T):bool $callable
+     *
+     * @return Option<T>
+     */
+    abstract public function filterNot($callable);
+
+    /**
+     * If the option is empty, it is returned immediately.
+     *
+     * If the option is non-empty, and its value does not equal the passed value
+     * (via a shallow comparison ===), then None is returned. Otherwise, the
+     * Option is returned.
+     *
+     * In other words, this will filter all but the passed value.
+     *
+     * @param T $value
+     *
+     * @return Option<T>
+     */
+    abstract public function select($value);
+
+    /**
+     * If the option is empty, it is returned immediately.
+     *
+     * If the option is non-empty, and its value does equal the passed value (via
+     * a shallow comparison ===), then None is returned; otherwise, the Option is
+     * returned.
+     *
+     * In other words, this will let all values through except the passed value.
+     *
+     * @param T $value
+     *
+     * @return Option<T>
+     */
+    abstract public function reject($value);
+
+    /**
+     * Binary operator for the initial value and the option's value.
+     *
+     * If empty, the initial value is returned. If non-empty, the callable
+     * receives the initial value and the option's value as arguments.
+     *
+     * ```php
+     *
+     *     $some = new Some(5);
+     *     $none = None::create();
+     *     $result = $some->foldLeft(1, function($a, $b) { return $a + $b; }); // int(6)
+     *     $result = $none->foldLeft(1, function($a, $b) { return $a + $b; }); // int(1)
+     *
+     *     // This can be used instead of something like the following:
+     *     $option = Option::fromValue($integerOrNull);
+     *     $result = 1;
+     *     if ( ! $option->isEmpty()) {
+     *         $result += $option->get();
+     *     }
+     * ```
+     *
+     * @template S
+     *
+     * @param S                $initialValue
+     * @param callable(S, T):S $callable
+     *
+     * @return S
+     */
+    abstract public function foldLeft($initialValue, $callable);
+
+    /**
+     * foldLeft() but with reversed arguments for the callable.
+     *
+     * @template S
+     *
+     * @param S                $initialValue
+     * @param callable(T, S):S $callable
+     *
+     * @return S
+     */
+    abstract public function foldRight($initialValue, $callable);
+}
diff --git a/src/PhpOption/Some.php b/src/PhpOption/Some.php
new file mode 100644
index 00000000..032632ea
--- /dev/null
+++ b/src/PhpOption/Some.php
@@ -0,0 +1,169 @@
+<?php
+
+/*
+ * Copyright 2012 Johannes M. Schmitt <schmittjoh@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace PhpOption;
+
+use ArrayIterator;
+
+/**
+ * @template T
+ *
+ * @extends Option<T>
+ */
+final class Some extends Option
+{
+    /** @var T */
+    private $value;
+
+    /**
+     * @param T $value
+     */
+    public function __construct($value)
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * @template U
+     *
+     * @param U $value
+     *
+     * @return Some<U>
+     */
+    public static function create($value): self
+    {
+        return new self($value);
+    }
+
+    public function isDefined(): bool
+    {
+        return true;
+    }
+
+    public function isEmpty(): bool
+    {
+        return false;
+    }
+
+    public function get()
+    {
+        return $this->value;
+    }
+
+    public function getOrElse($default)
+    {
+        return $this->value;
+    }
+
+    public function getOrCall($callable)
+    {
+        return $this->value;
+    }
+
+    public function getOrThrow(\Exception $ex)
+    {
+        return $this->value;
+    }
+
+    public function orElse(Option $else)
+    {
+        return $this;
+    }
+
+    public function ifDefined($callable)
+    {
+        $this->forAll($callable);
+    }
+
+    public function forAll($callable)
+    {
+        $callable($this->value);
+
+        return $this;
+    }
+
+    public function map($callable)
+    {
+        return new self($callable($this->value));
+    }
+
+    public function flatMap($callable)
+    {
+        /** @var mixed */
+        $rs = $callable($this->value);
+        if (!$rs instanceof Option) {
+            throw new \RuntimeException('Callables passed to flatMap() must return an Option. Maybe you should use map() instead?');
+        }
+
+        return $rs;
+    }
+
+    public function filter($callable)
+    {
+        if (true === $callable($this->value)) {
+            return $this;
+        }
+
+        return None::create();
+    }
+
+    public function filterNot($callable)
+    {
+        if (false === $callable($this->value)) {
+            return $this;
+        }
+
+        return None::create();
+    }
+
+    public function select($value)
+    {
+        if ($this->value === $value) {
+            return $this;
+        }
+
+        return None::create();
+    }
+
+    public function reject($value)
+    {
+        if ($this->value === $value) {
+            return None::create();
+        }
+
+        return $this;
+    }
+
+    /**
+     * @return ArrayIterator<int, T>
+     */
+    public function getIterator(): ArrayIterator
+    {
+        return new ArrayIterator([$this->value]);
+    }
+
+    public function foldLeft($initialValue, $callable)
+    {
+        return $callable($initialValue, $this->value);
+    }
+
+    public function foldRight($initialValue, $callable)
+    {
+        return $callable($this->value, $initialValue);
+    }
+}